For almost all of the evidence introduced in the Federal Grand Jury cases I heard, the witness was a law enforcement officer. They were typically the case lead

For almost all of the evidence introduced in the Federal Grand Jury cases I heard, the witness was a law enforcement officer. They were typically the case lead from a federal agency (Postal Inspectors, FBI, DEA, Secret Service, DHS, IRS, ATF, HUD, FDA, etc). They explain the documents that were collected through warrants and provide expert analysis.