One of my best friends was killed in a motorcycle accident in September '05.
He had about 12 guns in a safe. The house has been empty.  His two adult daughters got into the safe last Thanksgiving. Today they went into it again and all the guns are gone. A key and the combination was needed to get in. A M1A, 4 AR 15s, a Bullpup, a few shotguns, maybe 6 handguns. They just called me yesterday to say they were giving me an AR and another gun. My friend loved his guns, and it really bums me out that none of the family will have them now.
 They are calling the police. Will the Feds get involved? What kind of penalty would someone get if caught with the guns?
  I have the serial numbers of a couple of them. He bought them from me over the years.
  The M1A was bought from me when I had my Class III dealer license back in the '80s.

Possible Inside job. Are there any other relatives?

On the other hand, if the daughters had been in the safe and didn't close it correctly, someone could have gained access. I have had this happen to me on two occasions (nothing was stolen, gun safe and a fireproof filing cabinet both opened up when they were supposed to be locked).

Tough break.