Top U.S. congressman says shooter may have pledged allegiance to Islamic State militant group
* Officials probe whether attack inspired by Islamic State
* Carnage ends as police storm building, killing suspect
* Ex-wife says gunman aspired to be police officer
* Dealing with something "unimaginable" -mayor (Updates with comments from people who attended mosque with Mateen)
By Letitia Stein and Jarrett Renshaw
ORLANDO, Fla., June 12 (Reuters) - A man armed with an assault rifle and pledging loyalty to Islamic State killed 50 people during a gay pride celebration at a nightclub in Orlando, Florida, early on Sunday in the deadliest mass shooting in U.S. history, a rampage President Barack Obama denounced as an act of terror and hate.
Police killed the gunman, Omar Mateen, 29, a New York-born Florida resident and U.S. citizen who was the son of Afghan immigrants and was twice questioned by FBI agents in recent years, authorities said.
Mateen's former wife said he was emotionally and mentally disturbed with a violent temper, yet aspired to be a police officer.
Working for the global security firm G4S during the past nine years, Mateen was an armed guard for a gated retirement community in South Florida, and had cleared two company background screenings, the latest in 2013, according to G4S..
Preliminary investigations suggested the attack was inspired by Islamic State militants, though there was no immediate evidence that Mateen had any actual ties to the group, law enforcement officials said.
As the shooting rampage was unfolding, Mateen "made calls to 911 this morning in which he stated his allegiance to the leader of the Islamic State," said Ronald Hopper, the FBI's assistant special agent in charge on the case.
Shots rang out at the crowded Pulse nightclub in the heart of Orlando, about 15 miles northeast of the Walt Disney World Resort, as some 350 patrons were attending a Latin music event in conjunction with gay pride week celebrations. Clubgoers described scenes of carnage and pandemonium, with one man who escaped saying he hid under a car and bandaged a wounded stranger with his shirt.
"Words cannot and will not describe the feeling of that," Joshua McGill said in a posting on Facebook. "Being covered in blood. Trying to save a guy's life."
Fifty-three people were wounded in the rampage. It ranked as the deadliest single U.S. mass shooting incident, eclipsing the massacre of 32 people at Virginia Tech University in 2007.
"We know enough to say this was an act of terror, an act of hate," Obama said in a speech from the White House. "As Americans, we are united in grief, in outrage and in resolve to defend our people."
U.S. officials cautioned, however, they had no conclusive evidence of any direct connection with foreign extremists.
"So far as we know at this time, his first direct contact was a pledge of bayat (loyalty) he made during the massacre," said a U.S. counterterrorism official. "This guy appears to have been pretty screwed up without any help from anybody."
HEAVILY ARMED
The attacker was carrying an AR-15-style assault rifle and a handgun, Orange County Sheriff Jerry Demings said. He also had an unidentified "device," said Orlando Police Chief John Mina.
The shooting was nearly certain to reignite emotional debates over American gun laws and homeland security in what is shaping up to be a vitriolic U.S. presidential campaign between Democrat Hillary Clinton and Republican Donald Trump.
The attack came six months after a married couple in California - a U.S.-born son of Pakistani immigrants and a Pakistani-born woman he married in Saudi Arabia, fatally shot 14 people in San Bernardino in an attack inspired by Islamic State. That couple died in a shootout with police hours after their assault on a holiday party attended by the husband's co-workers.
The precise sequence of events in Orlando remained murky. But authorities said the gunman burst into the club and opened fire at about 2 a.m., then took dozens of hostages that he held at gunpoint inside a bathroom during a three-hour siege that ended when police stormed the building using armored cars and killed the gunman.
Officials in Orlando, a city of 270,000 people, were visibly shocked at the high death toll.
"We're dealing with something that we never imagined and is unimaginable," Orlando Mayor Buddy Dyer said. He said 39 people died inside the club, two outside, and nine others died after being rushed to the hospital.
Mateen had twice been interviewed by FBI agents, in 2013 and 2014, after making comments to co-workers indicating he supported militant groups, but neither interview led to evidence of criminal activity, the FBI's Hopper said.
Hopper said Mateen was questioned in 2014 about his contacts with Moner Mohammad Abu-Salha, a U.S. citizen who also had lived in Florida and became a suicide bomber in Syria that year.
Mateen and his family regularly attended a Florida mosque. "Not everyone had a friendship with him. He wasn't a people person. He was not extremely friendly but he wasn't rude either," said Mohammed Jameel, 54, a worshipper at the mosque.
Another worshipper said she recently saw Mateen dancing at his sister's wedding, a mixed affair that was open to both men and women.
STORMY MARRIAGE
Sunday night, federal agents combed through Mateen's apartment in the Atlantic coast town of Fort Pierce, about 120 miles southeast of Orlando, searching for clues, as numerous evidence vans sat parked outside.
"The FBI is currently in the apartment. They'll probably be there through the night," said St. Lucie County Sheriff Ken Mascara.
Near Boulder, Colorado, Mateen's former wife, Sitora Yusufiy, told reporters he worked for a time as a correctional officer at a juvenile detention center in Fort Pierce, and had once sought admission to a police academy.
She said she had been beaten by Mateen during outbursts of temper in which he would "express hatred towards everything." Eventually, she was "rescued" from Mateen by members of her family who intervened four months into 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 United States, 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 U.S. 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.
(Additional reporting by Barbara Liston in Orlando, Fla., Yara Bayoumy in Fort Pierce, Fla., Zachary Fagenson in Port St. Lucie, Fla., Colleen Jenkins in Winston-Salem, N.C., Keith Coffman in Denver and Jonathan Landay, Mark Hosenball, Ginger Gibson and Yeganeh Torbati in Washington; Writing by Scott Malone, Daniel Wallis, Steve Gorman and Roberta Rampton; Editing by Alistair Bell, Peter Cooney and Simon Cameron-Moore)
A police officer working as a security guard inside the Pulse nightclub, which has operated in downtown Orlando since 2004 and was hosting some 350 revelers, exchanged fire with the suspect at about 2 a.m. (0600 GMT), police officials said.
"Everyone get out of pulse and keep running," the club's management wrote on Facebook as the incident unfolded.
A hostage situation developed, and three hours later SWAT team officers used armored cars to storm the club before shooting dead the gunman. It was unclear when the victims were killed.
Dozens of terrified patrons, some of whom had been hiding in restrooms, were rescued. One officer was injured when he was hit in his helmet while exchanging fire with the gunman, police said.
Representative Adam Schiff, the top Democrat on a congressional intelligence committee, noted that the shooting took place during Ramadan, and that ISIS leaders in Syria have urged attacks during this time.
"The target was an LGBT nightclub during Pride, and - if accurate - that according to local law enforcement the shooter declared his allegiance to ISIS, (that) indicates an ISIS-inspired act of terrorism," Schiff said in a statement.
'DIFFERENT KIND OF CASE'
If confirmed as an act of terrorism, it would stand as the deadliest such attack on U.S. 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.
A pair of ethnic Chechen brothers killed three people and injured more than 260 with a pair of homemade pressure-cooker bombs at the Boston Marathon in April 2013
Asked if the FBI suspected the gunman might have had inclinations toward militant Islam, including a possible sympathy for Islamic State, Ronald Hopper, an assistant FBI agent in charge, told reporters: "We do have suggestions that the individual may have leanings toward that particular ideology. But right now we can't say definitively."
U.S. Senator Marco Rubio of Florida, who sits on the Senate intelligence and foreign relations committees, told CNN he understood that the gunman had worked for a security company and so would have undergone some background checks.
"Over the next couple of days they're going to be looking to see where this individual was inspired to carry out this horrifying act of terrorism," Rubio said. "I think we're going to be talking about a very different kind of case here soon."
President Barack Obama ordered the federal government to provide any assistance needed to Florida police investigating the shooting, the White House said. He was due to speak later Sunday.
The attacker was carrying an assault rifle and a handgun, Orange County Sheriff Jerry Demings said. He was also carrying an unidentified "device", Orlando Police Chief John Mina said earlier.
Video footage showed police officers and civilians carrying some people away from the club and bending over others on the ground. Dozens of police cruisers, ambulances and other emergency vehicles could be seen in the area.
'HORRIFIC ACT'
"It's definitely a tragedy not only for the city but for our entire nation," Mina told reporters. "We believe this is a tragedy that could happen anywhere in the United States of America, and what we need to do is continue to be vigilant."
Pulse, which was crowded for a Latin music night, describes itself on its website as more than "just another gay club." One of the founders and owners, Barbara Poma, opened it more than a decade ago in an effort to keep alive the spirit of her brother, who died after battling HIV.
The choice of target was especially heart-wrenching for members of the U.S. lesbian, gay, bisexual and transgender community, LGBT advocacy group Equality Florida said in a statement.
"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. "We will await the details in tears of sadness and anger."
Orlando has a population of 270,930 and is the home of the famed Disney World amusement park and many other tourist attractions that attracted 62 million visitors in 2014.
It was the second deadly shooting at an Orlando night spot in as many nights. Late Friday, a man thought to be a deranged fan fatally shot singer Christina Grimmie, a former contestant on "The Voice", as she was signing autographs after a concert.
(Additional reporting by Letitia Stein in Tampa, Colleen Jenkins in Winston-Salem, North Carolina, Chris Michaud in New York and Mary Milliken in Los Angeles; Writing by Frank McGurty, Scott Malone and Dan Wallis; Editing by Mark Heinrich and Mary Milliken)
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