"So far, we see no indication that this was a plot directed from outside the United States, and we see no indication that he was part of any kind of network," said FBI Director James Comey. But he said Mateen was clearly "radicalized," at least in part via the internet.
Comey said the bureau is also trying to determine whether Mateen had recently scouted Disney World as a potential target, as reported by People.com, which cited an unidentified federal law enforcement source.
"We're still working through that," Comey said.
The FBI chief defended the bureau's handling of Mateen during two previous investigations into his apparent terrorist sympathies. As for whether the FBI should have done anything differently, "so far the honest answer is, 'I don't think so,'" Comey said.
Wielding an AR-15 semi-automatic rifle and a handgun, Mateen opened fire at Pulse Orlando early Sunday in a three-hour shooting rampage and hostage siege. During the attack, he called 911 to profess allegiance to the Islamic State group.
At the White House, President Barack Obama said there is no clear evidence so far that Mateen was directed by the group, calling the attack an apparent example of "homegrown extremism."
More details of the bloodbath emerged, with Orlando Police Chief John Mina saying Mateen was "cool and calm" during phone calls with police negotiators. But the chief said he decided to send the SWAT team in and bash through a wall after Mateen holed up with hostages in a bathroom and began to talk about bombs and an explosive vest.
"We knew there would be an imminent loss of life," Mina said.
Five of the wounded were reported in grave condition, meaning the death toll could rise. A call went out for blood donations.
In Orlando, mourners piled bouquets around a makeshift memorial, and people broke down in tears and held their hands to their faces while passing through the growing collection of flowers, candles and signs about a mile from the site of the massacre.
About 300 employees of the Red Lobster restaurant chain - some in business suits, some in chef's uniforms - walked two-by-two across the street to the memorial, each carrying a red or white carnation.
"As the names come out, they are overwhelmingly Latino and Hispanic names," Christina Hernandez, a Hispanic activist, told a gathering. "It was Latino Night when this tragedy occurred. These were not just victims of the LBGT community, but of the Hispanic community, as well. This was senseless bloodshed."
Despite Mateen's pledge of fealty to the Islamic State, a murky combination of other possible motives and explanations emerged, with his ex-wife saying he was mentally ill - bipolar - and his Afghan-immigrant father suggesting he may have acted out of anti-gay hatred. He said his son got angry recently about seeing two men kiss.
Mateen's grasp of the differences between Islamic extremist groups appeared to be shaky.
During three calls with 911 dispatchers, Mateen not only professed allegiance to ISIS but also expressed solidarity with a suicide bomber from the Nusra Front, and a few years ago he claimed connections to Hezbollah, too - both ISIS enemies, according to Comey.
The FBI became aware of Mateen in 2013 when co-workers told agents about his comments about connections to al-Qaida and Hezbollah, Comey said.
The FBI launched a 10-month preliminary investigation but closed it after following him, reviewing his communications and questioning him, the FBI chief said.
His name surfaced again as part of another investigation into the Nusra Front bomber. The FBI found Mateen and the man had attended the same mosque and knew each other casually, but the investigation turned up "no ties of any consequence," Comey said.
On Sunday, the bloodshed started after Mateen approached the club around 2 a.m., exchanged fire with an off-duty officer working security, and then went inside and started gunning people down, police said.
After two other officers arrived and exchanged gunfire with Mateen, the gunman holed up in a restroom with about five club-goers. An additional 15 to 20 were in another nearby bathroom, authorities said.
Hostage negotiators began talking to Mateen.
After Mateen began to talk about explosives, Mina made the decision around 5 a.m. to blow open a wall to the bathroom. The explosives didn't penetrate the wall completely, so an armored vehicle was used to punch a 2-foot-by-3-foot hole. Dozens of people escaped, and Mateen was gunned down, police said.
The tragedy shocked the nation and cast a pall over Orlando, known all around the globe as the home of Walt Disney World and other theme parks.
"We will not be defined by the act of a cowardly hater," Mayor Buddy Dyer vowed.
The Islamic State's radio hailed the attack and called Mateen "one of the soldiers of the caliphate in America." But it gave no indication the group planned or knew of the attack beforehand.
Counterterrorism experts have been warning in the past few years about the danger of so-called lone wolf attackers who act in sympathy with extremist groups like the Islamic State but are not directed by them.
Mateen's father, Seddique Mir Mateen, told reporters that the massacre was "the act of a terrorist," and added: "I apologize for what my son did. I am as sad and mad as you guys are."
He wouldn't go into details about any religious or political views his son held, saying he didn't know. Asked whether he missed his son, he said: "I don't miss anything about him. What he did was against humanity."