Someone? Someone sent by Mr. Moore perhaps? Miss Marple, someone of your high intelligence and investigative chops should not be deceived by Moore's method of getting an item into the news media. Any PR hack in Hollywood can tell you that anonymous "someones" are often the best couriers.
I still want to know what's on that tape, and I would bet the FBI does, too.
If he gives it to Berg, there is no reason to subpoena him. Go after Berg instead, with a public announcement that although they will keep the tape from the public at the father's request, they must examine it to see if there are clues which would help them find Nick's killers.
Michael Berg would make no points refusing to release it to the FBI. People aren't going to believe that a grieving father would keep evidence from the FBI that might help solve the crime.