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.