WASHINGTON (AP) — He didn't come out of nowhere. The Orlando shooter came under suspicion three full years ago, after boasting of mutual acquaintances with the Boston Marathon bombers and making statements to co-workers that suggested he had radical, violent intentions. But after a 10-month investigation the FBI closed the case, finding no criminal charge to pursue.
Omar Mateen was scrutinized again in 2014 as part of a separate probe into a suicide bomber who attended the same Florida mosque and was a casual acquaintance. Again, agents found no significant radical ties and shifted their focus away.
Did investigators miss something they should have seen? A look at how law enforcement makes these life-and-death judgments:
DOES THE GOVERNMENT BELIEVE IT SHOULD HAVE DONE ANYTHING DIFFERENTLY?
"So far, the honest answer is, I don't think so," FBI Director James Comey says. But he also says the FBI will continue examining its past actions.
Mateen's behavior and contacts were enough to prompt surveillance, the use of confidential informants and three direct interviews with him. But his was just part of a huge volume of investigations that has only grown in an era of prolific Islamic State propaganda and its advocacy of do-it-yourself terrorism.
President Barack Obama on Monday lamented the difficulty of tracking "lone wolves" who operate without ties to known terror groups, and Comey said the FBI opens "hundreds and hundreds" of investigations across the United States just like the one Mateen faced.
Most conclude without any basis for arrest or further monitoring, and once that happens, there's no mechanism for keeping perpetual watch on those subjects — or preventing them from buying weapons. Mateen was added to a terror watch list when he came under investigation in 2013 but was removed once that matter was closed.
Discerning whether someone under scrutiny will commit violence, or even who should be scrutinized, is more than just finding the proverbial needle in a haystack, Comey said. "It's which pieces of hay are likely to become a needle."
HOW BIG IS THAT HAYSTACK, REALLY?
Obama said Monday it's concerning when radical groups are promoting violence "very effectively over the internet." In the U.S., he said, "out of 300 million, there are going to be some individuals who find for whatever reason that kind of propaganda enticing."
WHAT'S THE PROTOCOL FOR FBI COUNTERTERROR INVESTIGATIONS?
Guidelines set by the attorney general spell out different levels of investigation that permit agents to use progressively more intrusive methods depending on the circumstances.
Mateen's 2013 examination was a preliminary investigation, a type of inquiry that requires a supervisor's approval and can go on for six months, with an extension if warranted. This one lasted 10 months before being closed. In a preliminary investigation, agents may track phone calls and obtain Internet communications and bank records.
Investigators looking into Mateen's statements to co-workers introduced confidential informants, followed him and recorded his conversations, among other methods.
Mateen admitted making the statements reported by his coworkers, but explained that did it in anger because he thought they were discriminating against him and teasing him because he was Muslim, Comey said.
"The evidence developed during the investigation was consistent with his explanation that he had said these things to try to freak out his co-workers," Comey said. Agents ultimately accepted that explanation.
More serious, longer-term terrorism investigations can employ invasive techniques such as obtaining an order from the Foreign Intelligence Surveillance Court to conduct electronic surveillance and physical searches.
HOW OFTEN DO PEOPLE PREVIOUSLY KNOWN TO THE FBI GO ON TO COMMIT VIOLENCE?
It's certainly not unheard of.
A notable example is Tamerlan Tsarnaev, who along with his brother carried out the 2013 Boston Marathon bombings. The FBI interviewed Tsarnaev and relatives in 2011 following a tip from a Russian intelligence security service that he was a follower of radical Islam. The FBI did not find any domestic or foreign terrorism activity.
More recently, Elton Simpson, one of the two men fatally shot during an attempted attack last year on a Prophet Muhammad cartoon contest in Garland, Texas, had previously been prosecuted in Arizona in a terrorism-related case. He was given probation for lying to a federal agent. But soon before the May 2015 violence, he re-emerged onto the FBI's radar because of postings on social media.
DOES THE FBI KEEP TRACKING PEOPLE IT ONCE INVESTIGATED?
Investigations are routinely closed when agents conclude they have insufficient cause to keep them open. The FBI maintains records in case the name surfaces again in connection with another matter. But agents can't indefinitely maintain surveillance without new basis to suspect wrongdoing.
People who were once looked at for potential terror ties can be removed from watch lists and generally resume their lives. And, Comey said, once an investigation is closed, "there is no notification of any sort" triggered by the person later trying to buy a firearm.
"Every case has to come to an end someplace, and if you don't have evidence that somebody commits a crime, then you're obligated to close that case," said David Gomez, former counterterrorism chief of the FBI's Seattle field office.
ARE LOCAL AUTHORITIES KEPT IN THE LOOP?
Comey said he was confident local law enforcement was made aware of the FBI's findings and the fact that it was closing the investigation into Mateen.
Often times, information is shared directly with local authorities through their representatives on joint terrorism task forces.
The FBI was criticized in 2013 after the Boston Marathon attack for not sharing enough information about the Tsarnaev brothers with local law enforcement. A few months after the attack, the FBI's deputy director sent an email to special agents in charge around the country reminding them to consistently share specific information with local task force officers.