(CN) - Twenty-four hours after the worst shooting rampage in U.S. history, law enforcement officials said they still have more questions than answers about why Omar Mateen slaughtered 49 before losing his own life in a hail of bullets Sunday morning.
FBI Director James Comey told reporters attending a briefing in Washington Monday afternoon that Mateen had "strong indications of radicalization," and "potential inspiration by foreign terrorist organizations," but that there is still hard evidence his actions were directed by others.
Comey also said while Mateen expressed allegiance with the Islamic State during multiple 911 calls placed from inside the Pulse nightclub Sunday morning, he also aligned himself with Moner Mohammad Abu-Salha, a suicide bomber who had lived in Fort Pierce, Fla., but was devoted to fighting against ISIL.
But he went on to remind reporters that the investigation is only in its early stages and that there's still a long way to go.
"We will continue to look forward and backward in this investigation. We will leave no stone unturned. We are looking for needles in a nationwide haystack," he said.
Comey's remarks were reiterated by President Barack Obama, who said at this stage of the investigation there is no indication that Mateen was part of a larger plot.
"In that sense, it appears to be similar to what we saw in San Bernardino, but we don't yet know. And this is part of what is going to be important in terms of the investigation," the president said.
"As far as we can tell right now, this is certainly an example of the kind of homegrown extremism that all of us have been so concerned about for a very long time. It also appears that he was able to obtain these weapons legally because he did not have a criminal record that, in some ways, would prohibit him from purchasing these weapons," he continued.
The president acknowledged that Mateen had been a subject of FBI scrutiny, but he stood by the agency's decision not to investigate him further, saying agents "followed the procedures that they were supposed to and did a proper job."
U.S. Attorney Lee Bentley on Monday said investigators are continuing to collect electronic and physical evidence at two residences tied to the gunman.
"We don't know if anyone else will be charged," he said at an early morning news conference.
But he emphasized authorities believe there is no imminent danger to general public.
Also at Monday's briefing, Orlando police Chief John Mina offered more details about the mass shooting, which began shortly after 2 a.m. Sunday morning.
Mina said a uniform officer was working at the Pulse nightclub in full uniform when Mateen showed up at the door with an AR-15 semi-automatic rifle, and the two engaged in a brief gun battle.
Additional officers arrived at the club, and a second gun battle ensued, at which time Mateen retreated to a bathroom.
"At that time we were able to save and rescue dozens and dozens of people and get them out of the club," Mina said.
With that, a three-hour siege began with law enforcement negotiators speaking with Mateen by phone.
