Had agents sought a grand jury subpoena to obtain data from YouTube to identify the person behind the posting, it is not certain that prosecutors would have agreed to seek one based on the scant information available. Agents might have a hard time convincing prosecutors of an imminent threat because the post mentioned no time or location of a possible shooting.

Even if agents had tied Mr. Cruz to the YouTube post, the authorities probably would have questioned him or his family and friends but would have been unable seize his gun without a court order.

“I hope he would have been interviewed by the F.B.I. or referred to the local police assuming he was identified,” said Eugene Casey, a veteran former F.B.I. agent. “I would have done my best to identify the individual who made the threat, but he could have posted it to YouTube from a public computer in a library or somewhere else.”