Pretty standard investigation procedures. You've found potential EVIDENCE of a crime. Anyone who paid any attention to the O.J. Simpson trial knows the importance of preserving the integrity of evidence.
Law enforcement plays for breaks. All you need to start cracking a case is a tiny fissure where you can put a wedge and start hammering, and they're looking for cracks. Not necessarily with you, but anywhere. I guarantee they ran a background check on you before you left the field office. If they had any probable cause, they would have detained you. Some fools bring evidence of their own crimes to the police to divert suspicion from themselves if they think the heat is on. Or maybe the "good citizen" wants to burn a co-conspirator. Since law enforcement is paid to be suspicious, they're considering those possibilities. That dummy M-40 grenade you found won't look good....
Incidentally, every investigator knows the value of time. Time tends to change memories, and your story told today will differ from the story you tell in a month. Investigators seize on that fact to find inconsistencies in your story. Shoot, we lawyers do it all the time. It's how you discredit a witness. Don't be surprised if the FBI contacts you in a few weeks to ask you some more questions.
Look out for black helicopters and sedans.
