LAS VEGAS >> Investigators trying to figure out why Stephen Paddock gunned down 59 people from his high-rise hotel suite are analyzing his computer and cellphone, looking at casino surveillance footage and seeking to interview his longtime girlfriend.

Nearly two days after the deadliest mass shooting in modern U.S. history, what set off the 64-year-old high-rolling gambler and retired accountant remained a big question mark today.

While the probe into his background included searches of two houses he owned in Nevada, some investigators turned their focus from the shooter’s perch to the killing grounds outside the Mandalay Bay hotel-casino where his victims fell.

A dozen investigators, most in FBI jackets and all wearing blue booties to avoid contaminating evidence, entered the festival site where gunfire erupted Sunday night and country music gave way to screams of pain and terror.

“Shoes, baby strollers, chairs, sunglasses, purses. The whole field was just littered with things,” said Nevada Attorney General Adam Laxalt, who told The Associated Press it was like a “war zone.” ”There were bloodstains everywhere.”

Paddock killed himself before a SWAT team blew off the door of his room on the 32nd floor. He had 23 guns with him at the hotel — along with devices that can enable a rifle to fire continuously, like an automatic — and 19 more guns at one of his homes, authorities said.

More than 500 people were injured in the rampage, some by gunfire, some during the chaotic escape. At least 45 patients at two hospitals remained in critical condition.

Retired FBI profiler Jim Clemente speculated that there was “some sort of major trigger in his life — a great loss, a breakup, or maybe he just found out he has a terminal disease.”

Clemente said a “psychological autopsy” may be necessary to try to establish the motive for the attack. If the suicide didn’t destroy Paddock’s brain, experts may even find a neurological disorder or malformation, he said.

He said there could even be a genetic component to the slaughter: Paddock’s father was a bank robber who was on the FBI’s most-wanted list in the 1960s and was diagnosed a psychopath.

“The genetics load the gun, personality and psychology aim it, and experiences pull the trigger, typically,” Clemente said.

Paddock had a business degree from Cal State Northridge. In the 190s and ’80s, he worked as a mail carrier and an IRS agent and held down an auditing job in the Defense Department, according to the government. He later worked for a defense contractor.

He had no known criminal record, and public records showed no signs of financial troubles, though he was said to be a big gambler.

“No affiliation, no religion, no politics. He never cared about any of that stuff,” his brother, Eric Paddock, said outside his Florida home. He said he was at a loss to explain the massacre.

Nevada’s Gaming Control Board said it pass along records compiled on Paddock and girlfriend Marilou Danley to investigators. Danley is expected to speak with detectives when she returns to the U.S. from out of the country.

The FBI discounted the possibility of international terrorism early on, even after the Islamic State claimed responsibility for the attack.

Sally Ho and Brian Skoloff in Las Vegas; Brian Melley in Los Angeles; and Sadie Gurman and Tami Abdollah in Washington contributed to this report.