FBI Director James B. Comey said Monday there are "strong indications of radicalization" of the gunman in Sunday's massacre of 49 people at a gay club in Orlando due "at least, in some part, by the internet."
But Comey said that there was no evidence that the shooter, 29-year-old Omar Mateen, was part of a larger conspiracy or was directed by an extremist group like the Islamic State.
Comey said Mateen had spoken with a 911 operator three times after he had taken hostages in Orlando's Pulse nightclub and was in a standoff with police, and pledged allegiance to Islamic State leader Omar al-Baghdadi by name. Islamic State later claimed responsibility for the attack, calling Mateen a "fighter," but it's not clear the group ever had contact with him, much less supported or had prior knowledge of the attack.
Comey also said the FBI had monitored Mateen for 10 months in 2013 for possible terrorism connections, including using undercover operatives, but dropped the surveillance after it found no evidence.
Comey said one of the reasons for the monitoring was Mateen's relationship to a man who later became a suicide bomber with al-Qaeda's affiliate in Syria, the Nusra Front. Comey said Mateen attended the same mosque as Moner Mohammad Abu-Salha, who blew himself up in May in Syria.
During the phone calls with authorities in the middle of the shooting rampage, Mateen mentioned support for the leader of Islamic State, the Boston Marathon attackers, and a Florida man who became an al Nusra Front suicide bomber in Syria, Comey said. Al Nusra is an al Qaeda offshoot at odds with Islamic State.
"The bombers at the Boston marathon and the suicide bomber from Florida were not inspired by ISIL, which adds a little bit to the confusion about his motives," Comey said.
"We're working hard to understand the killer and his motives and his sources of inspiration but we're highly confident this killer was radicalized at least in some part through the internet," he said.
Comey said the FBI was looking hard at its own prior investigations of Mateen's behavior "to see if there is something we should have done differently.
"So far, the honest answer is: I don't think so," he said.
"We are looking for needles in a nationwide haystack but we're also called upon to find out which pieces of hay might someday become needles. That is hard work."
Reuters contributed to this report.