The suspect behind the deadliest mass shooting in US history was 'upset when he saw men kissing' and called 911 to pledge allegiance to IS before opening fire in a gay club, killing 50 people.
Omar Mateen, 29, opened fire inside popular gay bar Pulse in Orlando, Florida, and was shot on the scene after police stormed the building.
Police are now investigating whether Muslim American Mateen, Afghan descent, had a terrorist motive.
A police officer told NBC Mateen called 911 shortly before the attack and pledged allegiance to ISIS leader Abu Bakr al-Baghdad.
"During a conference call with federal law enforcement officials a short time ago, Massachusetts State Police and other local law enforcement authorities learned that the Orlando nightclub gunman, during his rampage, pledged allegiance to ISIS and referenced the Tsarnaev brothers," state police spokesman David Procopio wrote in an email.
The Tsarnaev brothers were responsible for killing three people and injuring more than 260 in the Boston Marathon attack on April 15, 2013.
US officials have emphasised they have no immediate evidence Mateen has any direct connection with Islamic State or any other foreign extremist group.
Mateen family members said he was not overly religious but was homophobic and regularly assaulted his ex-wife.
Sitora Yusufiy said she met Omar Mateen online about eight years ago and they decided to get married and move in together in Fort Pierce, Florida, in March 2009.
The marriage was normal at first, she says, but then he became abusive. She left him in 2011, fearing for her life.
"He was not a stable person," Yusufiy told the Washington Post.
"He beat me. He would just come home and start beating me up because the laundry wasn't finished or something like that."
The woman's parents later rescued her from the relationship and the pair were later divorced, according to a court document seen by AFP.
"They literally saved my life," she said.
Mateen was a "private person" who was not especially expressive about his Muslim faith, Yusufiy said.
She said his family was from Afghanistan, but he was born in New York.
Mateen's father, Mir Seddique, believes the shooting was not an act of terrorism.
"We are in shock like the whole country," Seddique said. "This had nothing to do with religion."
He said Mateen had recently been offended to see two gay men expressing affection on a Miami street.
"We were in downtown Miami, Bayside, people were playing music," the father told NBC News in the immediate aftermath if the shooting.
"And he saw two men kissing each other in front of his wife and kid and he got very angry," Seddique said.
The imam of the Florida mosque, Syed Shafeeq Rahman, said Mateen was a loner who kept to himself.
"He hardly had any friends," he said.