The US Second Circuit court of appeals just ruled on a very important case about Fourth Amendment protections for seized computer files. While this ruling is only binding on courts in the 2nd circuit, it will be influential, and we are likely to see this issue addressed by the Supreme Court before too long.
The reality of computer forensics is that investigators start by grabbing everything off the computers they are searching, then look for the specific information specified in the warrant. Generally this is done by making a direct image of the computer’s hard drive. From there additional copies are made so the chain of evidence is clean, and the original image can be shown to be unchanged. It is impractical to try to capture only the targeted information because the volumes are often so large the search must be automated and may take considerable time. Additionally, suspects may have taken steps to try to hide files on the disks.
The upshot of this is that the law enforcement entity now has a great many documents far outside the scope of the warrant. This is where we come to the specifics of the case United States v. Ganias. In 2003 the government searched Ganias’ computers as part of a fraud investigation. As I described, they captured full images of all the computer’s hard drives to 19 DVDs. After competing their searches, they kept the DVDs.
In 2006, they thought Ganias might be involved in tax related crimes, so they obtained warrants to search the DVDs they had in storage for this different set of documents.
The 2nd Circuit ruled to suppress the evidence obtained from that 2006 warrant because the documents searched should never have been seized in the first place.
The ruling recognizes the realities of the search process, and allows for capture of full drive images, and keeping that data for a reasonable time, but specifically forbids keeping it indefinitely as a source of information in future searches. That would completely void the Fourth Amendment which requires that the warrant specify the specific things to be searched.
As a reminder, the full text of the Amendment is:
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
Thanks to the Washington Post for a more detailed legal analysis: Court adopts a Fourth Amendment right to the deletion of non-responsive computer files - The Washington Post