FBI gives its version of how the Orlando shooter slipped through

The FBI investigated Orlando shooter Omar Mateen on multiple fronts and even put him on a government watch list for a time but eventually closed the inquiries because there wasn't enough evidence to connect him to terrorism, FBI Director James Comey said.

That's why no alarms went off earlier this month, when Mateen bought the guns that he used to kill 49 people in Sunday's shooting spree at a gay dance club in Orlando, Florida, the FBI chief said Monday at a news conference in Washington.

The Federal Bureau of Investigation will "look at our own work, to see if there is something we should have done differently," Comey said. "So far, I think the honest answer is I don't think so. We will continue to look forward in this investigation and backward."

While the FBI is treating the massacre in Orlando as a terrorist act, a central point of contention is whether investigators missed vital clues about Mateen in the past. It's also raising pressure on U.S. officials as they struggle to detect lone terrorists without violating civil liberties.

"I don't see anything in reviewing our work that our agents should have done" differently, Comey told reporters.

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Mateen, who was killed in a shootout with police, called 911 as he began the assault to claim allegiance to Islamic State, and the terrorist group issued a statement afterward praising the attack.

Comey said the FBI opened a preliminary investigation into Mateen that lasted 10 months, from May 2013 to March 2014, after co-workers in his job as a contract security guard at a courthouse raised concerns that he had terrorist leanings. Agents used multiple tactics, including having confidential sources talk with him, reviewing his electronic communications and interviewing him twice, Comey said.

In two interviews with the FBI, Comey said, Mateen indicated that he made statements about connections with terrorists "because he thought his co-workers were discriminating against him and teasing him because he was Muslim."

Mateen was put on a government watch list during the inquiry but was removed shortly after it concluded, Comey said. He declined to further describe the list. Mateen wasn't on the government's no-fly list of people barred from boarding a commercial aircraft into or out of the U.S., according to an official familiar with the matter who asked not to be identified.

FBI agents questioned Mateen again two months later because they learned he had a brief association with Moner Mohammad Abusalha, the first American suicide bomber to die in Syria. Agents concluded the contact was minimal and the inquiry never led to a new investigation, Comey said.

"We don't keep people under investigation indefinitely," Comey said.

Comey said there are no indications at this time that Mateen's attack was directed by a terrorist group and that the 29-year-old appeared radicalized in part by propaganda on the internet.

Mateen didn't appear to have a coherent allegiance to any particular group, Comey said, "which adds a little bit to the confusion about his motives." He professed support for different terrorists, from Islamic State to al-Nusra and Hezbollah — all of which are at odds with each other. Comey also said the agency was trying to "understand what role anti-gay bigotry may have played in motivating this attack."

"Our work is very challenging," Comey said. "We are looking for needles in a nationwide haystack, but we're also called upon to determine which pieces of hay might someday become needles. If we can find a way to do that better, we will."

The FBI has to have a valid reason to continue an investigation, such as the potential threat to life, said a U.S. official, who asked not to be identified discussing agency practices.

Law enforcement agencies don't have the resources and often can't show a judge probable cause to keep under surveillance the thousands of people that come onto their radar, said the official.

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While the U.S. has made progress in countering groups like Islamic State on the ground overseas, technology allows their radical ideology to reach across borders and lure true believers, the socially disaffected or the mentally unstable. Even when a potential terrorism suspect comes to the attention of U.S. law enforcement — as Mateen did — there may not be enough evidence, resources or coordination to continue an investigation.

Rep. Michael McCaul, R-Texas, head of the House Homeland Security Committee, told CNN that the FBI argues that it didn't have "what's called predication and evidence under the Constitution to properly contain, arrest, and charge this individual."

"Is it frustrating? Yes," McCaul said. "Are we going to be look at that and asking questions about what happened? Of course."

Comey said he believes the FBI has the resources it needs to counter plots. Others, however, said the magnitude of the terrorist problem leaves the bureau having to constantly juggle priorities.

"Law enforcement is following hundreds of people and there are thousands of people that have come on their radar," Shawn Henry, a former FBI executive assistant director, said in an interview. "The complexity of trying to navigate our laws and Constitution while trying to maintain optimal security is a really difficult challenge. You just cannot protect against everything."

Senior U.S. national security officials, including Comey, have warned for several years about the threat from "lone-wolf" terrorists — those who become self-radicalized and plot attacks with little notice or resources.

The challenge for intelligence and law enforcement officials, however, is knowing when radical beliefs have crossed the line into action.

"The step from just having some extreme views to acting violently is extremely hard to detect from the outside unless that individual is sharing that with others on social media, telephonically, or what have you," said Daniel Benjamin, director of the Dickey Center at Dartmouth College and a former State Department counterterrorism coordinator.

That effort is compounded in a country like the U.S., where free speech and the ability to buy guns are considered fundamental rights. "Just making statements isn't enough to arrest somebody," Republican Sen. Marco Rubio of Florida, a member of the Intelligence Committee, said on CBS News.

With assistance from Nafeesa Syeed, Eltaf Najafizada, Alan Levin and Mike Dorning.