Mateen was born in New York, NBC News reports.
He was married in 2009, public records show. It is not clear if he was still married at the time of the attack.
Mateen is a registered Democrat who has also lived in Fort Pierce, Florida, according to online records.
Florida Senator Bill Nelson says, according to intelligence committee staff, Mateen has ties to ISIS.
Mateen called 911 at the time of the attack to pledge allegiance to ISIS, a U.S. official tells CNN, and mentioned the Boston bombers in the call. He had been investigated by the FBI in the past for possible ties to Islamic extremism, officials say, but there was no evidence to charge him with anything. ISIS sympathizers have praised the attack on jihadi forums.
Siddiqui is a family name or surname meaning "The Truthful" in Arabic. Siddiquis are spread all over the world mainly Asia Pacific and Europe. Siddiquis are generally considered to be the descendants of the first Muslim Caliph Abu Bakr as-Siddiq of the Rashidun Caliphate (The four Rightly guided Calips).
Mateen is a Muslim name meaning "powerful, and thus, for example, variants like the name Abdul Mateen is a Muslim name meaning, "servant of the firm," "strong."
The day after singer Christina Grimmie died from an Orlando shooting, a Florida gay club in the same city has erupted in gunfire. The Orlando club named Pulse was the 2 a.m. target of a long-gun attack and possible hostage situation early Sunday, June 12, 2016.
Multiple injuries are reported. "Mass casualty situation" is what it is being called. Some reports said 250 have being hit, with at least 42 have been sent to hospitals.
"Everyone get out of Pulse and keep running," the club posted on its Facebook page.
Pulse describes itself as "the hottest gay bar" in the heart of Orlando. Hours before the shooting, the club urged partygoers to attend its "Latin flavor" night Saturday.
"People on the dance floor and bar got down on the floor and some of us who were near the bar and back exit managed to go out through the outdoor area and just ran," he posted on the club's Facebook page.
Other notable June 12 events:
* 1994 – Nicole Brown Simpson and Ronald Goldman are murdered outside her home in Los Angeles, California. O.J. Simpson is later acquitted of the killings, but is held liable in wrongful death civil suit.
* 1963 – Civil rights leader Medgar Evers is assassinated in front of his home in Jackson, Mississippi by Ku Klux Klan member Byron De La Beckwith.
* 1943 – Holocaust: Germany liquidates the Jewish Ghetto in Brzeżany, Poland (now Berezhany, Ukraine). Around 1,180 Jews are led to the city's old Jewish graveyard and shot.