The 29-year-old, who died in a shootout with police, lived in Fort Pierce, Florida. Bomb squad teams, FBI agents and police combed his apartment there for hours on Sunday. Investigators haven't revealed what they found.
They also haven't said what led Mateen to attack the Pulse nightclub, which bills itself as "the hottest gay bar in Orlando."
A 911 call could offer a clue. Mateen called dispatchers about 20 minutes into the attack, pledging allegiance to ISIS and mentioning the Boston Marathon bombers, a U.S. official said.
Sunday's attack, officials said, wasn't the first time Mateen crossed paths with authorities.
Ex-wife: He abused me
The shooter's ex-wife, Sitora Yusufiy, held a press conference on Sunday. She said she believed Mateen — who she said abused her periodically during their relationship -- was emotionally unstable and suffered from mental illness.
"In the beginning he was a normal being that cared about family, loved to joke, loved to have fun, but then a few months after we were married I saw his instability. I saw that he was bipolar and he would get mad out of nowhere. That's when I started worrying about my safety," she said.
Yusufiy, who met Mateen online about seven years ago, said the abuse she suffered at Mateen's hands became so horrific that her family was forced to "rescue her," and she described the night she left all her belongings and fled.
"He started abusing me physically, very often, and not allowing me to speak to my family, keeping me hostage from them," Yusufiy said.
"[My family] had to pull me out of his arms and find an emergency flight. ... I made a police report."
FBI investigated him before
His full name was Omar Mir Seddique Mateen and he was born in New York. His parents are originally from Afghanistan, a U.S. official said.
The FBI had interviewed him in two terror-related cases, but both of them were closed, Assistant Special Agent in Charge Ronald Hopper told reporters Sunday.
"Those interviews turned out to be inconclusive," Hopper said, "so there was nothing to keep the investigation going."
Mateen was not under investigation or under surveillance at the time of Sunday's shooting, Hopper said.
"The FBI first became aware of Mateen in 2013 when he made inflammatory comments to co-workers alleging possible terrorist ties," Hopper said.
The investigation was closed, Hopper said, after investigators "were unable to verify the substance of his comments."
In 2014, the FBI interviewed Mateen again over possible connections with an American suicide bomber.
"We determined that contact was minimal and did not constitute a substantive relationship or threat at that time," Hopper said.
Officials earlier said Mateen was one of hundreds of people on the agency's radar suspected of being ISIS sympathizers. There was no indication he was plotting to carry out an attack, the officials said.
Armed with handgun, assault rifle
The gunman was armed with a handgun, an assault rifle and an unknown number of rounds when he attacked, Orlando Police Chief John Mina told reporters.
Both weapons were legally purchased "within the last few days," ATF Assistant Special Agent in Charge Trevor Velinor told reporters.
Mateen worked as a security officer at G4S Secure Solutions, one of the world's largest private security companies, which said Sunday it was "shocked and saddened" by the news
"We are cooperating fully with all law enforcement authorities, including the FBI, as they conduct their investigation," G4S Secure Solutions said.
Authorities evacuated about 200 people from the Fort Pierce apartment complex where Mateen lived as they investigated the scene Sunday, police said.
A neighbor told CNN Mateen worked as a security guard at the courthouse in Port St. Lucie, Florida. Mateen often worked at the front of the building, staffing the metal detectors, according to the neighbor.
Authorities are looking into Mateen's electronic devices for any suspicious activity and trying to put together a timeline
of his movement before the attack, law enforcement officials said.
Police are also investigating the suspect's vehicle, a van parked outside the nightclub, Mina said. Mateen rented a car and drove to Orlando to carry out the attack, a law enforcement source told CNN.
Outrage over two men kissing
A message posted on a site associated with the ISIS news agency Amaq described the attacker as "an Islamic State fighter." But the language is inconsistent with previous ISIS announcements, and there was no claim the attack was directed, just an after-the-fact assertion the gunman was an ISIS fighter.
The shooter's family told investigators he wasn't particularly religious.
They expressed surprise about any connection to ISIS, according to two law enforcement officials.
Officials stressed that their investigation is in the early stages. They say they're looking into the possibility Mateen radicalized on his own. In addition to possible ties to the terror group, there could be other factors at play.
After seeing two men kissing in Miami, Mateen expressed outrage to his father, the officials said.
His ex-wife told investigators he had anger issues, according to one of the officials.