It is mind boggling that they let this man go. How stupid can they be? I guess they forgot that the Constitution was not meant to be a suicide pact. It is ridiculous that non-citizens of the U.S. are automatically given constitutional rights. They should have found some reason to hold this man for a few days, at the very least, while they investigated his background more thoroughly.
I think the FBI should have just participated in the instruction and thereby it would have allowed them to find out more about what the guy had planned. Besides, they could have given him misinformation; couldn't they?