LAS VEGAS – A gunman in a high-rise hotel overlooking the Las Vegas Strip opened fire on a country music festival late Sunday, killing at least 59 people and injuring hundreds of others in the deadliest mass shooting in modern American history.

The gunman, identified by police as Stephen Paddock, was later found dead by officers on the 32nd floor of the Mandalay Bay Resort and Casino, Sheriff Joseph Lombardo said during a news briefing Monday.

The massacre marked the nation’s latest outbreak of gunfire and bloodshed to erupt in a public place, again spreading terror in an American city transformed into a war zone. The carnage in Las Vegas surpassed the 49 people slain in June 2016 when a gunman in Orlando, who later said he was inspired by the Islamic State, opened fire inside a crowded nightclub.

Lombardo said investigators could not immediately identify a motive, leaving no clear answer as to why a gunman gunned down dozens of people. He also said an additional 527 people were injured, though he did not specify how many were wounded by gunfire or injured in the chaos that followed. Clark County Fire Chief Greg Cassell said officials were seeing a “wide range” of injuries including gunshot victims as well as people injured by shrapnel, trampling or getting hurt jumping fences attempting to escape.

Paddock, 64, was found dead in his hotel room by Las Vegas SWAT officers, police said. They believe Paddock, who had checked in on Thursday and brought a cache of guns, took his own life in Room 135 before officers entered.

Under the neon glow and glitz of the Vegas Strip, thousands of concertgoers who had gathered for a three-day music festival dove for cover or raced toward shelter when the gunfire began at about 10 p.m. Sunday. Police said more than 22,000 people were at the concert when Paddock began firing round after round, shooting from an elevated position that left those on the ground effectively helpless. The typical advice for reacting to an active shooter – “run, hide or fight” – was rendered moot, as many in the packed crowd could not easily run or hide, nor were they able to fight back at someone firing from so far away.

Police described Paddock as a “lone wolf” attacker. Lombardo did not give further details on Paddock’s background and possible motivation, saying that police “have no idea what his belief system was.”

“I can’t get into the mind of a psychopath,” Lombardo said during a later briefing Monday. He also said that given the belief that Paddock was a lone attacker, “I don’t know how this could have been prevented.”

Police said Paddock smashed his room’s window – which would appear to those below to be gold – with something similar to a hammer before he began to fire, pumping off rounds that were audible in rooms floors away. Paddock was found with more than 10 rifles, Lombardo said, and he brought them all inside himself.

Authorities found a small arsenal inside Paddock’s hotel room. Officers found about 20 firearms, according to people familiar with the case. One of the weapons he apparently used in the attack was an AK-47-style rifle, with a stand used to steady it for firing, these people said.

Lombardo said authorities at the gunman’s home found more than 18 guns as well as explosives and “several thousand rounds of ammo,” along with unspecified electronic devices they were evaluating.

Gun purchase records suggest Paddock legally bought more than two dozen firearms over a period of years, according to a person close to the probe.

At least some guns he owned were purchased at Guns & Guitars, a Mesquite, Nevada, store. The general manager of the store on Monday said Paddock was “a customer and purchased firearms from our store; however, all necessary background checks and procedures were followed, as required by local, state and federal law.”

Investigators are working to determine if Paddock modified any of his long guns to function as fully automatic – capable of spitting out a high volume of fire just by holding down the trigger, according to people familiar with the probe, who asked not to be identified.

Because Paddock was a high roller at the Vegas casinos, investigators are also scouring his records to try to determine if he was facing financial strains, according to these people.

One official familiar with the investigation said the scene inside the hotel room suggested Paddock had spent a lot of time meticulously planning the attack – particularly for someone with no known background in weaponry. The official pointed to a number of factors suggesting the foresight and deliberation that went into the attack – bringing a large quantity of weapons and ammunition, a tool to knock out the hotel room window before firing and keeping all of that equipment out of sight of hotel staff until he was ready to carry out the attack.

Relatives of Paddock’s said they were stunned by what happened. Paddock had retired and lived in Mesquite, Texas, for several years before moving to the Nevada town with the same name. Relatives described him as a quiet man, a licensed pilot who liked to gamble. His brother, Eric, said their mother spoke to the FBI.

“She said, ‘I don’t understand why my son did this,’ ” Eric Paddock said Monday morning outside his home in Orlando, Florida. While his brother had some handguns, Eric Paddock but was shocked by the weaponry police described in Las Vegas.

Eric Paddock said he did not know of his brother having any mental illness, alcohol or drug problems. When he spoke to the FBI, Eric Paddock said he showed FBI agents three years of text messages from his brother, including one that mentioning winning $250,000 at a casino. Stephen Paddock played “high stakes video poker,” Eric said, adding that he did not have any information suggesting the 64-year-old gunman had gambling debts or financial issues.

Their father, Benjamin, was one of the FBI’s most-wanted fugitives decades ago, a convicted bank robber described in one wanted poster as “psychopathic” with suicidal tendencies. Eric Paddock told reporters that Benjamin, their father, was not around much during their childhood.

A former neighbor of Stephen Paddock’s recalled that his home in a 55-and-over community in Florida looked like it was home to college freshman, with nothing on the walls and only a few pieces of furniture.

“One of the first times we met him, he told me he lived there, in Vegas,” Don Judy, his next-door neighbor in the community until two years ago, recalled. “He explained that he was a gambler, and a prospector. He said he was buying this house to check it out for his mother . . . and that if she liked it, he planned to buy another next door with a floor-plan like ours.”

Just as quickly as he appeared, Judy said, Paddock put up a for-sale sign and was gone, saying that he was moving back to Las Vegas.

As Las Vegas police investigated the horror that had unfolded on the Strip, they also faced a tragedy within their own ranks. The dead included an off-duty city police officer, the department said Monday morning. Two other officers who were on duty were injured, police said; one was in stable condition after surgery, and the other sustained minor injuries.

“It’s a devastating time,” Lombardo said at one of the news briefings he held.

In the initial chaotic aftermath of the shooting, authorities sought a woman named Marilou Danley, described only as Paddock’s “traveling companion.” Lombardo said during a briefing that investigators spoke with Danley, who was found outside the country, and do not believe she was involved in the shooting, though she remained a person of interest.

Her relationship with Stephen Paddock was not immediately known, but they lived at the same address in Mesquite, Nevada, according to public records. Lombardo said police in Mesquite searched Paddock’s home on Monday.

Police in Las Vegas had only minimal interactions with Paddock before the shooting, Lombardo said.

“We have no investigative information or background associated with this individual that is derogatory,” the sheriff said. “The only thing we can tell is he received a citation several years ago; that citation was handled as a matter of normal practice in the court system.”

A spokesman for defense giant Lockheed Martin said in a statement that Paddock worked for the company for three years in the 1980s.

“Stephen Paddock worked for a predecessor company of Lockheed Martin from 1985 until 1988,” the statement said. “We’re cooperating with authorities to answer questions they may have about Mr. Paddock and his time with the company.”

On Monday President Donald Trump praised the “miraculous” speed with which local law enforcement responded to the shooting, which he decried as an unfathomable attack on innocents gathered for a concert.

“It was an act of pure evil,” Trump said during remarks from the White House. “We cannot fathom their pain, we cannot imagine their loss.”

Trump ordered flags flown at half-staff and said he would visit Las Vegas on Wednesday.

Federal homeland security officials said there were no specific, credible threats to other public venues around the country, while federal agents headed to Las Vegas to support the local police leading the investigation.

The Bureau of Alcohol, Tobacco, Firearms and Explosives said it dispatched agents and began “conducting urgent traces on firearms” recovered after the shooting. FBI criminal investigators – rather than those in the bureau’s National Security Branch – are also helping local police in the case, according to a person familiar with the matter.

The Islamic State claimed responsibility for the attack on Monday in multiple messages through its Amaq News Agency. In the messages, the group said the shooter was one of its “soldiers” and had recently converted to Islam, according to the SITE Intelligence Group, which tracks extremist groups.

The Islamic State, also known as ISIS, often claims responsibility after such attacks, even in cases where it is unclear whether the group motivated them or was involved. Law enforcement officials on Monday disputed the claims from ISIS.

“We have determined, to this point, no connection with an international terrorist group,” Aaron Rouse, the special agent in charge of the FBI in Las Vegas, said at a news briefing.

Attorney General Jeff Sessions said he met with FBI Director Christopher A. Wray on Monday morning and spoke with Lombardo, expressing him gratitude and offering federal support.

“The investigation into the horrific shooting last night in Las Vegas is ongoing,” Sessions said. “To the many families whose lives have been changed forever by this heinous act, we offer you our prayers and our promise that we will do everything in our power to get justice for your loved ones.”

After the shooting, scores of people gathered to donate blood to those in need in Las Vegas.

The shooting occurred at the end of the Route 91 Harvest festival, a three-day country music concert held over the weekend. The concert grounds are adjacent to the Mandalay Bay, a sprawling casino on the southern end of the Strip.

The shots began as Jason Aldean, one of the final performers, was playing. Aldean posted an Instagram message that he and his crew were safe. The scene, he wrote, was “beyond horrific.”

Videos posted from people who said they were at the scene showed people screaming and running for cover amid the sound of gunshots that seemed unending. “We thought it was fireworks at first or trouble with the speakers,” said Kayla Ritchie, 21, of Simi Valley, California. “They had been having technical difficulty all weekend. Then everything went dark.”

Ritchie traveled with Megan Greene, 19, for the concert, and the two were separated when people began fleeing. They found each other hours later. “Everyone started running for the exit,” said Greene, who hid behind a truck before running into the MGM Grand. “We were in the street, and they told us to get down, get down.”

Taylor Benge, 21, was at the concert Sunday night and said he heard a round of pops that lasted for 10 seconds, as if someone was holding down the trigger. When a performer ran off the stage and the lights came on, Benge said, he realized that “about five feet to the left of me there was a man with a bullet wound to his chin.”

“He was just lifeless on the ground,” Benge said.

At least some people were injured in the frenzied effort to flee the gunfire. Tracy, 55, a California woman who declined to give her last name, said she was “trampled” trying to flee.

“We thought it was fireworks,” she said, a dazed look on her face and a bandage on her injured knee and shin. “We ran for our lives. We went into Hooters and hid in the bathroom. We felt like sitting ducks there. We went to the second-floor conference room and stayed there.”

A friend came with a mini bus, so Tracy and another friend ran out to the vehicle, terrified to go out on the street again. “Who thinks people would do something like this in America?” Tracy said.

Corianne Langdon, 58, a cabdriver in Las Vegas for the last 6 ½ years, said she was about seven cars back in the taxi line at Mandalay Bay when the gunfire began and began to drive away, seeing police crouching in the street and hundreds of people running away from the concert – some jumping the fence on the side of the venue.

“I had people hanging out of my windows,” Langdon said. “They were screaming, they were so upset, and it just wasn’t getting to me yet the severity of what was going on.”

Those injured in the shooting also included an off-duty officer with the Bakersfield Police Department in Southern California, who was taken to a hospital for nonlife-threatening injuries, according to a news statement. Several of the department’s officers were off duty and attending the concert when the gunfire erupted.

The shooting came as security measures at many music venues have been boosted in recent years after concerts were targeted in terrorist attacks. In May in northern England, a bomb exploded at a concert by American singer Ariana Grande in Manchester, killing 22 people; in November 2015, Islamist attackers opened fire at a rock concert in Paris as part of coordinated attacks that left 130 dead. In both of those cases, the Islamic State claimed responsibility for the attacks.

In Manila in June, a 42-year-old Filipino gunman armed with a rifle and a bottle of gasoline burst into a casino and set a fire, killing 37 people. Police said the attack was motivated by gambling debts and other personal problems facing the gunman, who fatally shot himself. The Islamic State also claimed responsibility for that attack, but officials repeatedly denied it was terrorism-related.

—

Berman reported from Washington. The Washington Post’s Travis M. Andrews, Brian Murphy, Wesley Lowery, Devlin Barrett, Matt Zapotosky, Julie Tate and Aaron C. Davis in Washington; Barbara Liston in Orlando; and Dan Michalski in Las Vegas contributed to this report, which will be updated throughout the day.