What happens to Comey’s documents and to investigative files that have already been gathered? Can they be destroyed?
Remember that the FBI is a law enforcement agency. Not that Comey’s office is exactly a crime scene (yet), but the culture is one that places a high value on preserving information, not destroying it. Particularly in light of a letter from Democrats on the House Judiciary Committee requesting that all documents pertaining either to Comey’s firing or to the investigation into Russia’s election interference be preserved and put off-limits to White House officials and associates (as well as Attorney General Jeff Sessions, specifically), the FBI is under an affirmative duty to comply, and a failure to do so could be considered obstruction of justice. In general, any official documents that were in Comey’s office relating to the Russia investigation, such as memos regarding investigative steps or conclusions, approvals for decisions taken and communications with field offices would become part of the case file itself. Personal notes, emails and informal communications would likely be compartmentalized, classified if necessary, and remain in the custody of acting FBI Director Andrew McCabe.
FBI investigative files are official Department of Justice documents and as a legal matter, cannot be destroyed. (One of J. Edgar Hoover’s legacies is that he never got rid of anything: The National Archives contains FBI files going back to 1908, including up to 17,000 pages of documents maintained by Hoover himself in his personal vault.) This would be especially true for a case that has two active congressional intelligence committee investigations underway.
As a practical matter, it would not be possible to destroy FBI documents anyway, since case files are electronic and not paper-based—so there can be no “accidental” fires in the file room. Files are also kept in a secure system that tracks all access and is designed to prevent unauthorized tampering such as alterations or removal, so you can step down from the ledge: Everything will remain intact.