The FBI is reviewing its contacts with Orlando shooter Omar Mateen, but so far hasn't found any lapse in its dealings with him, Director James Comey said Monday.
"We will continue to look forward in this investigation and backward," Comey said in a nationally televised statement in Washington. "We will leave no stone unturned and we will work all day and all night to understand the path to that terrible night."
Mateen, 29, used an AR-15-style assault rifle in the massacre that left 49 people dead at the Pulse nightclub in Orlando, Florida, early Sunday. Authorities said he legally purchased the weapon last week. The New York-born shooter also was killed in a shootout with police that ended the three-hour siege. The attack was the worst mass shooting in modern U.S. history.
Hank Shaw, head of the FBI's field office in Boston, said in a statement to NBC News that "During one of the 911 calls between the operator and Omar Mir Seddique Mateen, Mateen made a reference to the Tsarnaev brothers by calling them his 'homeboys.' At this point in time, all evidence collected to date shows no connection between Mateen and the Tsarnaev brothers."
Comey said the FBI will consider its own work, "to see whether there's something we should've done differently." Comey pledged that the agency will examine its work in a transparent manner even though it hasn't found any missteps.
"Our work is very challenging. We are looking for needles in a nationwide haystack. But we're also called upon to figure out which pieces of hay might some day become needles. That is hard work. If we can find a way to do that better, we will," he said.
Comey said the agency first came into contact with the killer in May 2013 because of some "inflammatory and contradictory" statements he made about terrorism.
The killer had claimed family connections to Al-Qaeda and membership of Hezbollah, a Shiite organization that Comey noted is a "bitter enemy" of the so-called Islamic State, whose members are Sunni.
"He said that he hoped law enforcement would raid his apartment and assault his wife and child so that he could martyr himself," the FBI director said.
Following these reports from Mateen's co-workers, the FBI's Miami office opened an investigation, Comey said. The director added that Mateen admitted to making the statements because of perceived discrimination from his co-workers. The agency closed the investigation after 10 months, according to Comey.
The FBI came across Mateen again in July 2014, in a separate investigation of a "Florida man who had blown himself up for the Nusra front in Syria," Comey said.
"We learned from the investigation that the killer knew [the bomber] casually from attending the same mosque in that area of Florida, but our investigation turned up no ties of any consequence between the two of them," he said.
On Monday, President Barack Obama, speaking to reporters at the White House, said that the FBI had followed protocol when Mateen was being watched by the agency.
Obama said the massacre was "a devastating attack on all Americans" and that what happened in Orlando is an example of the "home-grown extremism that all of us have been so concerned about for a very long time."
"We are also going to have to make sure that we think about the risks we are willing to take by being so lax in how we make very powerful firearms available to people in this country," Obama said.
The president added that while it is important to continue to "go after these terrorist organizations and hit them hard," the U.S. can do more to prevent weapons from falling into the wrong hands.
"We make it very easy for individuals who are troubled or disturbed or want to engage in violent acts to get very powerful weapons very quickly and that's a problem," he said.
— NBC News' Tom Winter and CNBC's Matt Belvedere contributed to this report.