Carpenter Decision and IP-based Investigations in Digital Forensic Practice

Introduction

In Part 1 of this series, we walked through how subpoenas, pen registers, and IP address lookups support subscriber-level investigations. In Part 2, we examined how law enforcement uses peer-to-peer (P2P) network forensics and tools like Torrential Downpour to capture shared files on BitTorrent swarms.

This final installment focuses on the Carpenter decision and how it affects IP-based investigations built on BitTorrent evidence in child sexual abuse material (CSAM) cases. Only law enforcement investigators conduct CSAM BitTorrent investigations. Defense forensic experts become involved later, when the investigation and supporting data already exist, and their job is to examine, verify, and, when appropriate, challenge what law enforcement did.

The Carpenter decision has become a central reference point in arguments about digital privacy and surveillance. This article explains the ruling, shows how courts have treated Carpenter in BitTorrent CSAM cases so far, and highlights open questions about large-scale Internet Crimes Against Children (ICAC) dragnet monitoring of BitTorrent swarms and subsequent ISP subscriber lookups.

I am a digital forensics expert, not an attorney. Nothing in this series should be taken as legal advice. My goal is to explain the technical side of investigations and highlight how courts have handled these issues so far.

The Carpenter decision as a digital privacy precedent

The Carpenter decision comes from Carpenter v. United States, 138 S. Ct. 2206 (2018). The Supreme Court held that when the government acquires historical cell-site location information (CSLI) from a wireless carrier, that acquisition is a search under the Fourth Amendment and generally requires a warrant supported by probable cause [1]. The Court emphasized the “depth, breadth, and comprehensive reach” of CSLI and how it enables “near perfect surveillance” of a person’s movements over time.

In doing so, the Carpenter ruling carved out an exception to the traditional third-party doctrine. Before Carpenter, courts often held that if a suspect voluntarily shared information with a company (phone numbers dialed, bank records, etc.), then they lacked a reasonable expectation of privacy in those records. Carpenter v. United States limited that logic for highly revealing, long-term location data.

Because of this, commentators describe Carpenter as a digital privacy precedent that invites courts to reexamine how the Fourth Amendment applies to modern digital surveillance techniques, including IP-based investigations and large-scale monitoring of online activity [2].

For digital investigators, Carpenter raises an important question: when does metadata, including IP addresses and session logs, become so comprehensive and revealing that it triggers Carpenter’s warrant requirement? That question sits at the center of the emerging IP-based forensics standard.

How CSAM BitTorrent investigations work (law enforcement side)

In the CSAM context, law enforcement uses P2P tools to monitor BitTorrent swarms for known contraband. Torrential Downpour, for example, is a law enforcement-only Child Protection System (CPS) program that searches the BitTorrent network for hash values of files already identified as CSAM and attempts to download those files directly from a single IP address [3].

A simplified law enforcement workflow in a CSAM BitTorrent case looks like this:

1. The investigator loads one or more target torrent hashes—corresponding to known CSAM—into Torrential Downpour or a similar CPS tool.
2. The program searches public BitTorrent swarms for peers advertising those hashes and logs the IP addresses and ports that appear to offer the files.
3. The program attempts a “single-source” download from one IP address at a time and records the communication logs (timestamps, infohash, file paths, and other metadata).
4. Once a successful download completes and the file hashes match known CSAM, the investigator issues a subpoena to the ISP to identify the subscriber assigned that IP address at the relevant time.
5. Based on the downloaded material, the logs, and the subscriber information, law enforcement applies for a search warrant and executes it at the physical location associated with the subscriber account.
6. Forensic examiners then image seized devices and correlate artifacts (BitTorrent clients, logs, and CSAM files) with the network evidence.

At each stage, the investigator generates technical artifacts: Torrential Downpour logs, ISP subscriber records, and device-level forensic data. Those artifacts are the core materials that a defense forensic expert will later review.

Importantly, in BitTorrent CSAM cases, only law enforcement personnel run Torrential Downpour or similar CPS tools against live swarms. Defense experts do not run parallel CSAM searches on the live network; they analyze how the original investigation was conducted, using the data and logs that already exist.

What defense forensic experts do (after the fact)

Defense forensic experts step in after the law enforcement investigation and warrant execution. Their role is fundamentally different from that of a law enforcement analyst:

• They do not initiate BitTorrent CSAM investigations or connect to swarms.
• They do not run Torrential Downpour against live targets.
• Instead, they examine the digital record of what law enforcement already did.

Typical tasks for a defense forensic expert include:

• Reviewing Torrential Downpour logs to verify which IP address was targeted, which files were requested, and what the tool actually downloaded.
• Confirming that the file hashes downloaded by law enforcement match the hashes on the seized devices, or identifying gaps where they do not match.
• Examining the BitTorrent client configuration and logs on seized devices to determine whether the device could have been the source Torrential Downpour connected to.
• Reviewing ISP records, including dynamic lease information and any network address translation (NAT) or carrier-grade NAT details that may weaken the link between IP address and account holder.
• Comparing the technical record against the warrant affidavit to see whether the affidavit accurately described the technology and evidence.

Defense experts also help counsel assess whether a Carpenter-based argument is plausible. For example, does the investigation involve a one-time connection to a publicly shared file, or does it involve long-term, large-scale tracking of IP addresses across many swarms and time periods? That distinction is central to the emerging peer-to-peer evidentiary shift.

How courts have applied Carpenter to BitTorrent CSAM cases

Defendants in several CSAM BitTorrent cases have tried to use the Carpenter decision to suppress evidence obtained through Torrential Downpour and similar tools. So far, courts have remained unconvinced that downloading files or metadata from a publicly shared BitTorrent client falls within Carpenter’s protection.

United States v. Hoeffener (Eighth Circuit)

In United States v. Hoeffener, the Eighth Circuit considered a challenge to Torrential Downpour in a BitTorrent CSAM case [3]. The court held that the defendant had no legitimate expectation of privacy in files he made available to the public through a P2P file-sharing network, even after Carpenter. The panel emphasized that Torrential Downpour only downloaded files that the defendant’s BitTorrent client had already offered publicly, and that the software did not access non-public areas of the computer.

This reasoning fits a line of cases that treat P2P file sharing as functionally equivalent to placing contraband on a public website for anyone to download. Under that view, no search occurs when law enforcement downloads what any member of the public could have downloaded.

Youngman v. State (Florida Second DCA)

In Youngman v. State, the Florida Second District Court of Appeal addressed Torrential Downpour in a state CSAM case [4]. The court described Torrential Downpour as a Child Protection System tool “available only to law enforcement” that automates the process of searching for hash values of known CSAM on BitTorrent. The opinion emphasized that the software “merely automates the aggregation of public information,” a task that investigators could have carried out manually by connecting to the swarm and downloading the same files.

Youngman reinforces the idea that, as long as law enforcement only accesses files that have been voluntarily shared with the public, no new Fourth Amendment search occurs, even when an automated tool performs the work.

United States v. Carme (District of Massachusetts)

In United States v. Carme, the defendant argued that the use of Torrential Downpour and subsequent seizure of his devices violated Carpenter because the investigation allegedly built a detailed profile of his digital activity [5]. The district court rejected this argument. It concluded that the Carpenter decision did not apply where law enforcement downloaded files the defendant had made available on a public P2P network and then used a standard subpoena and warrant process to identify and search his devices.

The Carme court also noted that a more elaborate explanation of BitTorrent and Torrential Downpour would not have changed the probable cause analysis, and it declined to treat the P2P investigation as a Carpenter-type search.

Recent appellate decisions

More recently, the Eleventh Circuit in United States v. Ewing joined the Eighth Circuit’s approach, holding that the government did not conduct a Fourth Amendment search when it used Torrential Downpour to download child pornography from a BitTorrent user [6]. The court cited United States v. Hoeffener approvingly and agreed that a suspect has no reasonable expectation of privacy in files shared on a public P2P network.

These decisions show a consistent trend: courts have remained unconvinced that BitTorrent sharing of torrent files publicly deserves privacy protection under Carpenter. When a user configures their BitTorrent client to share files with anyone on the swarm, courts view that choice as fundamentally different from the involuntary, pervasive tracking at issue in Carpenter v. United States.

The unresolved question: ICAC dragnets and ISP account-holder information

While courts have largely rejected Carpenter-based challenges to law enforcement downloads of files from public BitTorrent swarms, a different question remains less tested: how Carpenter might apply to the large-scale ICAC dragnet of BitTorrent swarm monitoring and subsequent ISP subscriber lookups.

In many jurisdictions, ICAC task forces run Torrential Downpour or related CPS tools continuously. These programs may log vast numbers of IP addresses, timestamps, and hash values across many swarms and long time periods. The technical capability exists to build detailed histories of which IP addresses appeared in which swarms, advertising which hashes and when.

So far, courts have tended to treat each IP lookup and download as a discrete event, framing the investigation as “public file download plus simple subscriber identification under the third-party doctrine.” Cases like Hoeffener and Youngman focus on the fact that law enforcement only downloaded files the user shared publicly and then used routine legal process to obtain subscriber data from the ISP.

Based on current decisions, courts have remained unconvinced that public BitTorrent sharing itself triggers Carpenter. However, in light of the massive ICAC dragnet of BitTorrent swarm monitoring, whether the ISP account-holder information linked to that long-term, large-scale data collection should require a search warrant may not be fully tested. Commentators at organizations like NACDL’s Fourth Amendment Center have argued that Carpenter’s logic may apply to certain forms of aggregated third-party data that reveal a person’s “digital travels, personal curiosities, and online associations” over time [2].

This is where the peer-to-peer evidentiary shift intersects with the Fourth Amendment: the difference between a snapshot of public activity (Hoeffener/Ewing) and a surveillance mosaic of a user's life (Carpenter). A one-time download from a public swarm looks very different from years of continuous monitoring of the same IP address across thousands of torrents, followed by repeated subpoenas for subscriber information.

Practical implications for law enforcement analysts

For law enforcement investigators who conduct CSAM BitTorrent operations:

• Treat Torrential Downpour as a public-download tool, not a remote-search tool. Keep the use of the software strictly limited to files and metadata that a BitTorrent user has shared publicly. Make sure policies and training reflect this boundary, consistent with cases like Hoeffener and Youngman.
• Maintain detailed logs. Preserve logs showing which torrents were targeted, which IP addresses responded, how the single-source downloads occurred, and what was actually downloaded. Those logs will be central when defense experts review the investigation.
• Document the subpoena and warrant trail. Clearly record how subscriber information was obtained, what legal process was used, and when warrants were sought. This record helps show that ISP subscriber data was accessed through traditional third-party doctrine mechanisms rather than Carpenter-type mass location tracking.
• Avoid unnecessary long-term tracking of specific IPs. Although current case law is favorable to law enforcement, the more the investigation begins to look like longitudinal tracking of an IP address across many swarms, the stronger future Carpenter-based arguments may become. Minimizing unnecessary retention and focusing on case-specific activity can reduce risk.
• Coordinate with prosecutors on emerging law. The Carpenter ruling continues to evolve in lower courts. Investigators should work closely with prosecutors to adapt investigative practices as the IP-based forensics standard develops.

Practical implications for defense forensic experts

Defense forensic experts, by contrast, operate downstream. They do not run Torrential Downpour or initiate CSAM BitTorrent investigations, but they play a vital role in evaluating them:

• Reconstruct the investigation timeline. Use Torrential Downpour logs, subpoenas, and warrants to build a precise timeline of law enforcement actions: when the tool connected, what it downloaded, when subscriber information was requested, and when the warrant was executed.
• Validate technical claims. Compare the hashes and file names in the Torrential Downpour logs with files found on seized devices. Look for inconsistencies, gaps, or signs that the downloaded files could have originated from a different host behind carrier-grade NAT or a shared network.
• Assess the scope of monitoring. Determine whether the investigation focused on a short window and a few downloads, or whether it involved extended monitoring across many swarms and timestamps. That assessment helps counsel decide whether a Carpenter-based argument about “depth, breadth, and comprehensive reach” is plausible.
• Support legal arguments about third-party data. While the case law currently disfavors privacy claims in publicly shared files, the status of ISP subscriber records in the context of large-scale ICAC monitoring is less clear. Defense experts can help attorneys articulate how the combination of prolonged swarm monitoring and subscriber lookups may resemble the digital surveillance patterns Carpenter sought to constrain.
• Explain complex technology to the court. Judges often need clear technical explanations to understand how BitTorrent works, what Torrential Downpour does, and where its limits are. A careful explanation, grounded in logs and reproducible analysis, can influence how the court applies the Carpenter decision in the specific case.

Summary

The Carpenter decision reshaped the Fourth Amendment analysis for certain kinds of third-party digital data, especially historical location records. In CSAM BitTorrent investigations, however, courts have repeatedly held that defendants lack a reasonable expectation of privacy in files they share publicly on P2P networks. Cases such as United States v. Hoeffener, Youngman v. State, and United States v. Carme show that the Carpenter ruling has not persuaded courts to suppress evidence obtained when law enforcement uses Torrential Downpour to download publicly offered files and then subpoenas ISPs for subscriber information.

At the same time, the digital privacy precedent set by Carpenter v. United States continues to influence how courts and commentators think about large-scale, long-term monitoring of online activity. In the ICAC context, the question of whether extensive BitTorrent swarm monitoring combined with ISP account-holder lookups should require a warrant under Carpenter has not been fully tested.

Law enforcement analysts who run CSAM BitTorrent investigations must document their work carefully and respect current limits, while defense forensic experts must scrutinize those investigations after the fact, translating complex technical records into arguments that fit within an evolving IP-based forensics standard.

Call to action

If you are a criminal defense attorney handling a CSAM BitTorrent case, or a defense forensic expert reviewing a Torrential Downpour investigation, you do not need to navigate these technical and legal issues alone. Lucid Truth Technologies can help reconstruct the investigation, analyze the evidence, and prepare clear, courtroom-ready explanations that account for both the Carpenter decision and current case law on peer-to-peer evidence.

References

[1] Supreme Court of the United States, Carpenter v. United States, 138 S. Ct. 2206, 2018. [Online]. Available: https://www.supremecourt.gov/opinions/17pdf/16-402_h315.pdf

[2] M. Price and B. Wolf, “Building on Carpenter: Six New Fourth Amendment Challenges Every Defense Lawyer Should Consider,” National Association of Criminal Defense Lawyers (NACDL), 2018. [Online]. Available: https://www.nacdl.org/Content/Building-on-Carpenter-Six-New-Fourth-Amendment-Cha

[3] United States Court of Appeals for the Eighth Circuit, United States v. Hoeffener, 950 F.3d 1037, 2020. [Online]. Available: https://law.justia.com/cases/federal/appellate-courts/ca8/19-1192/19-1192-2020-02-24.html

[4] District Court of Appeal of Florida, Second District, Youngman v. State, 342 So. 3d 770, 2022. [Online]. Available: https://caselaw.findlaw.com/court/fl-district-court-of-appeal/2178390.html

[5] United States District Court for the District of Massachusetts, United States v. Carme, No. 1:19-cr-10073, 2020. [Online]. Available: https://www.govinfo.gov/content/pkg/USCOURTS-mad-1_19-cr-10073/pdf/USCOURTS-mad-1_19-cr-10073-0.pdf

[6] United States Court of Appeals for the Eleventh Circuit, United States v. Ewing, No. 24-11308, 2025. [Online]. Available: https://law.justia.com/cases/federal/appellate-courts/ca11/24-11308/24-11308-2025-06-23.html