Orlando Mayor Buddy Dyer, second from right, and Orlando Police Chief John Mina arrive to a news conference after a fatal shooting at Pulse Orlando nightclub in Orlando. -AP
Hopper said Mateen was questioned in 2014 about his contacts with Moner Mohammad Abu-Salha, a US citizen who also had lived in Florida and became a suicide bomber in Syria that year.
Near Boulder, Colorado, Mateen's former wife, Sitora Yusufiy, told reporters he worked for a time as a correctional officer at a detention centre for juvenile delinquents in Fort Pierce, Florida, and had once sought admission to a police academy.
She said she had been beaten and otherwise physically abused by Mateen during outbursts of temper in which he would "express hatred towards everything".
Eventually, she was “rescued” from Mateen by family members who intervened in a stormy marriage that ultimately ended in divorce, she said.
“I know he had a history of steroids,” Yusufiy told reporters outside a home where she was staying with a man she identified as her current fiance. She also described Mateen as “emotionally unstable”, “mentally ill” and bipolar.
Deborah Sherman, an FBI spokeswoman in Denver, confirmed that federal agents had interviewed Yusufiy in Colorado.
The imam of the Florida mosque where Mateen attended prayers for nearly 10 years described him as a soft-spoken man who would visit regularly but rarely interact with others in the congregation.
Candidates weigh in
Within hours of the shooting, the presumptive presidential nominees of both major political parties weighed in with statements on the tragedy.
Trump, who has called for a temporary ban on Muslims entering the US, said he was “right on radical Islamic terrorism” and called on Obama to resign because he did not say the words “radical Islam” in his statement responding to the shooting.
Clinton echoed Obama's comments calling the attack both an act of terror and a hate crime, adding that the massacre “reminds us once more that weapons of war have no place on our streets".
If confirmed as an act of terrorism, it would be the deadliest such attack on US soil since Sept 11, 2001, when Al Qaeda-trained hijackers crashed jetliners into New York's World Trade Center, the Pentagon and a field in Pennsylvania, killing some 3,000 people.
The choice of target was especially heart-wrenching for members of the US lesbian, gay, bisexual and transgender community, said LGBT advocacy group Equality Florida.
“Gay clubs hold a significant place in LGBTQ history. They were often the only safe gathering place and this horrific act strikes directly at our sense of safety,” the group said in a statement.
“We will await the details in tears of sadness and anger.”
In an apparently unrelated incident on Sunday, a heavily armed man from Indiana who said he was headed to a Los Angeles-area gay pride festival was arrested in nearby Santa Monica, California, where police found guns and chemicals to make explosives in his car.