The shooter in Sunday night's mass slaughter of 59 concert goers in Las Vegas, identified as 64-year-old Stephen Craig Paddock, a man with no criminal history, is now being investigated for possible ties to Islamic terrorists.

At least 527 people were injured in what was described by Sheriff Joseph Lombardo as a well-coordinated attack.

ISIS claimed credit in an Arabic-language website, but authorities immediately tried to debunk the claim, saying they had no evidence to support it.

"We will investigate that down to the end," but thus far nothing has been found to connect Paddock to international terrorism, Lombardo said at one of four news conferences held Monday on the shooting.

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"We have determined, to this point, no connection to an international terrorist group," said an unidentified spokesman for the FBI in a brief statement at a news conference at 11:40 a.m. Monday in Las Vegas.

The questions stemmed from a Twitter posting by the group Site Intel Group that ISIS spokesman Amaq has claimed Paddock was one of its “soldiers” who converted to Islam months ago.

BREAKING: #ISIS claimed #LasVegas attack, reporting through 'Amaq that executor is one of its "soldiers" & he converted to Islam months ago pic.twitter.com/4E85vEIUzH — SITE Intel Group (@siteintelgroup) October 2, 2017

The statement, in Arabic, read, "Attacker of the #Las_Vegas shooting is a soldier of the Islamic State and carried out the operation in response to targeting coalition countries," Jihadoscope, a cybermonitoring company that monitors jihadi activity, confirmed in a translation sent to Newsweek.

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Michael S. Smith II, co-founder of terror analysis company Kronos Advisory, who regularly tracks ISIS activity, also confirmed to Newsweek the claim of responsibility and the claim about Paddock's conversion.

After media continued to discredit the ISIS claims, the terrorist organization doubled down by sending out an official communique identifying Paddock as "Abu Abdul Bar al-Amriki."

This is the first U.S. attack that ISIS has claimed responsibility for in 2017.

Steven Stalinsky, executive director of Middle East Media Research Institute, noted the language posted on the group's Amaq news agency site matched the language of previous claims that turned out to be true.

Stalinsky says when an attack is carried out by someone with no formal connection to ISIS, the individual is typically referred to as a "soldier" who has often pledged allegiance in an email or online to the terrorist group.

"I am reserving judgment," Stalinsky told the Washington Examiner. "But in the past when they claim an attacker is a soldier, somehow there has generally been a connection."

Paddock reportedly had no military background, leading to questions about where he learned how to modify the AR-15s he used into a fully automatic weapons. He could have purchased them illegally from someone else who modified the guns. But given that he built his own platforms from which to shoot down into the crowd, it's not out of the question that he converted the gun himself.

The shooting took place at the Route 91 Harvest Festival about 10:08 p.m. Sunday. Country music star Jason Aldean was on stage with approximately 22,000 people in attendance when the bullets started raining down on the crowd, sparking a mass stampede out of the kill zone that caused dozens more to be injured.

The shooting began as the last act was performing on the last day of the event. The attack on the Ariana Grande concert in the U.K. earlier this year also took place at the end of the event.

Police seized 19 additional guns, ammonia nitrate and Tannerite from Paddock's Mesquite, Nevada, home, the sheriff's office reported at a 6 p.m. press conference. They also found thousands of rounds of ammo. Tannerite is a brand name for a patented exploding target used in firearms practice, while ammonia nitrate could have been part of a bomb-making operation.

The modified AR-15s used in the attack reportedly fired .223 and .308 caliber rounds capable of piercing police armor.

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Who is Stephen Paddock?

Stephen Craig Paddock, 64, hunkered down in his hotel room stocked with 17 guns and thousands of rounds of ammunition carried into the hotel in suitcases. Police found an additional 19 guns, several thousand rounds of ammo and explosive devices inside one of Paddock's three homes. He was a multi-millionaire real-estate investor and retired accountant who once worked for the defense giant Lockheed Martin.

Paddock used two windows on the 32nd floor of the Mandalay Bay Resort hotel as his turrets. He had set up cameras to monitor first responders and fired through the hotel door at security guards. He had checked into the hotel Thursday and set up two shooting platforms. He surveilled the area he planned to attack for three days before launching his massive firepower on the crowds.

The attack was deemed the worst shooting in modern U.S. history, surpassing the Orlando Pulse nightclub attack, which killed 49 people on June 12, 2016.

The shooter had no criminal background, said Lombardo. He is said to have killed himself as police closed in on him.

Paddock reportedly worked as an auditor and was employed for at least two years in the late 1980s by Lockheed Martin. He was a licensed hunter and private pilot, having obtained his pilot's license in November 2003, but hadn't updated his medical certification to fly since 2008.

Neighbors said Paddock was extremely unfriendly and disappeared for long stretches at a time.

The Vegas shooter "disappeared for 6 months last year," according to his neighbor. Was he radicalized and trained in heavy weapons then? — Bill Mitchell (@mitchellvii) October 2, 2017

He had previously lived in Melbourne and Orlando, Florida, but retired to Las Vegas because he loved to gamble, his brother told Fox News.

Paddock has also lived in Reno and Melbourne, Florida, and Henderson, Nevada, as well as locations in Texas and California since 1990.

Paddock’s father was Patrick Benjamin Paddock, a convicted bank robber who was on the FBI’s Most Wanted List.

His home at 1372 Babbling Brook Court in Mesquite is in an upscale Sun City retirement community. The home's value is estimated at $417,819. He bought it for $369,022 on Jan. 20, 2015, according to Trulia.

Paddock was reportedly estranged from his ex-wife of six years, whom he divorced 27 years ago. They have had no contact in many years, according to reports.

Paddock broke out two windows in the hotel room, apparently with hammer, through which he aimed his guns.

There were no obstructions, an absolute open-view shot into what became a massive kill zone.

"He knew what he was doing and he had clearly scoped out this area," Daniel Linskey, former Boston police superintendent, told Fox News. "This will change hotels going forward looking at security and training of their staff. It's a nightmare to have an elevated position. In Boston we always said 'imagine how much worse it would have been if along with the bomb you had people shooting those running out.'"

The concert was in its final night of a three-day event.

Paddock was a man of means. He owned two aircraft, according to the "Today" show. He lived in the suburbs around Mesquite, Nevada, which sits near the border with Arizona about an hour-and-a-half from where the shooting took place, police said.

Police in Mesquite said Paddock is a mystery to them because they have never had cause to have interaction with him. They never called to his house and were not sure how long he even lived in their city.

Police released a photo of Marilou Danley, a woman described as Paddock's live-in girlfriend, whom they consider a "person of interest" in the crime.

By 3:30 a.m. Pacific Monday, police said they had located Danley outside the country in Tokyo and were expecting to interview her upon her return.

The gunman's brother, Eric Paddock, told reporters for the Las Vegas Review outside his Central Florida home early Monday "an asteroid just fell on us," and he said Stephen Paddock has no history of mental illness.

Retired FBI special agent and former national FBI spokesman John Iannarelli, who drove past the scene of the event just moments before the shooting took place, told Fox News the massacre was "obviously well planned."

Iannarelli added the gunman used “expensive guns and ammunition.”

Iannarelli noted that FBI and police are going to speak with every friend and relative, and they are conducting forensic review of the suspect's computers and phones.

Sunday's shooting came more than four months after a suicide bombing at an Ariana Grande concert in Manchester, England, that killed 22 people. Almost 90 people were killed by gunmen inspired by Islamic State terror group at the Bataclan concert hall in Paris during a performance by Eagles of Death Metal in November 2015.