WASHINGTON — Last fall, a bail bondsman in Mississippi spotted a disturbing comment on his YouTube channel.
“Im going to be a professional school shooter,” it read. There was nothing more.
The bondsman, Ben Bennight, took a screenshot and flagged the comment to YouTube, which removed the post. Mr. Bennight then left a voice mail message at his local F.B.I. field office alerting it to the comment.
A pair of agents interviewed him the next morning, Mr. Bennight said in an interview on Thursday. But he heard nothing more until Wednesday, hours after the school shooting in Parkland, Fla., that killed 17: The commenter’s YouTube user name, “nikolas cruz,” matched that of the suspect.
Two F.B.I. agents visited Mr. Bennight and questioned him for 15 to 20 minutes. They told him they thought the person who posted on his channel might be connected to the Florida shooting because they had the same name. Nikolas Cruz, 19, has been charged with 17 counts of premeditated murder and has confessed to the killings, the police said Thursday.
“I really don’t feel like I was any help to them at all,” said Mr. Bennight, 36.
The F.B.I. is reviewing what steps the bureau took before the shooting when the agents received the initial information. The bureau conducted database reviews and open-source checks about the comment in September, Rob Lasky, the F.B.I. special agent in charge in Miami, said on Thursday.
Agents are still trying to sort out what happened, but without more information and limited ability under the circumstances to obtain a subpoena, their options were few.
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.”
The case highlights the difference between how the United States government handles terrorism and other kinds of attacks. Over the past decade and a half, the F.B.I. and other agencies have established broad power to pre-emptively investigate people in matters of international terrorism — raising expectations that the government should be able to prevent attacks.
“If they had gotten a tip that someone was going to shoot up the embassy in Cairo, they would have been all over it. But they don’t know what to do with it when it’s domestic,” said Chris Fialko, a prominent North Carolina defense lawyer who has handled numerous F.B.I. investigations. “We are not Big Brother yet. They have to get search warrants. In America, the Bill of Rights still applies.”
The F.B.I. is gathering information about Mr. Cruz’s social media posts, along with his movements and conversations, Mr. Lasky said. YouTube did not respond to a request for further information.
Mr. Bennight said he did not fault the F.B.I. for failing to stop the shooting.
“We live in a country where you can’t just lock people away for saying something,” he said. “You can’t just stuff somebody in a black hole because they said something that makes you uncomfortable. I believe the F.B.I. took it seriously. I hope that they followed up.”
If the bureau missed an opportunity to prevent the shooting, it could find itself in the cross hairs of Congress, which has criticized the F.B.I. for failing to stop the 2013 Boston Marathon bombing and the 2009 mass shooting at Fort Hood in Texas. In the Texas attack, the shooter was known to the F.B.I., as was one of the brothers who carried out the Boston bombing.