Investigators are looking into whether gunman Stephen Paddock scoped out bigger music festivals in Las Vegas and Chicago — and perhaps Boston's Fenway Park — before setting up his perch in a casino hotel and raining deadly fire on country music fans.

Paddock booked rooms overlooking the Lollapalooza festival in Chicago in August and the Life Is Beautiful show near the Vegas Strip in late September, according to authorities reconstructing his movements before he undertook the deadliest mass shooting in modern US history.

It was not clear if he contemplated massacres at those sites.

Investigators looking into Paddock also came across mention of Fenway Park, Boston police Lieutenant Detective Mike McCarthy said, though he provided no further details.

The details came to light as investigators struggled to figure out why the high-stakes gambler opened fire on a crowd of 22,000 Sunday night from the 32nd floor of the Mandalay Bay hotel casino in Las Vegas. He killed 58 people and injured nearly 500 before taking his own life.

A federal official said authorities are looking into the possibility Paddock planned additional attacks, including a car bombing. The official wasn't authorised to discuss the matter publicly and spoke to the Associated Press on condition of anonymity.

Authorities previously disclosed Paddock had 1,600 rounds of ammunition in his car, along with fertiliser that can be used to make explosives and 50 pounds of Tannerite, a substance used in explosive rifle targets.

In the meantime, Las Vegas police announced Thursday that they had found a Hyundai Tucson they had been searching for as part of the probe while executing a search warrant at the home in Reno that Paddock shared with his girlfriend, Marilou Danley. It wasn't immediately clear if the car was found on Thursday or earlier in the week when police searched the home and found several guns and ammunition.

Paddock had an arsenal of 23 weapons in his hotel room. A dozen of them included “bump stocks,” attachments that can effectively convert semi-automatic rifles into fully automated weapons.

In a rare concession on gun control, the National Rifle Association (NRA) announced its support on Thursday for regulating the devices.

Danley told FBI agents Wednesday she had not noticed any changes in his mental state or indications he could become violent, the federal official said.

Paddock sent Danley on a trip to her native Philippines before the attack, and she was unaware of his plans and devastated when she learned of the carnage while overseas, she said in a statement.

Investigators combing through his background for clues remain stumped as to his motive.

The profile developed so far is of a “disturbed and dangerous” man who acquired an arsenal over decades, Sheriff Joseph Lombardo said. But investigators have been frustrated to find that he lived a “secret life,” Lombardo said, “much of which will never be fully understood.”

A former executive casino host at the Atlantis Casino Resort and Spa in Reno said Paddock had a “god complex” and expected quick service without regard to how busy the staff was at the time.

“He liked everybody to think that he was the guy,” John Weinreich said. “He didn't boast about anything he had or anything. It was just his demeanour. It was like, 'I'm here. Don't cross me. Don't look at me too long.'”

The weekend before the massacre, he rented a room through Airbnb at the 21-storey Ogden condominiums in downtown Las Vegas and stayed there during a music festival below that included Chance the Rapper, Muse, Lorde and Blink-182.

“Reasons that ran through Paddock's mind is unknown, but it was directly at the same time as Life Is Beautiful,” the sheriff said.

Police were reviewing video shot at the high-rise to check Paddock's movements. His renting the condo was curious because, as a high-roller, he could have easily gotten a free room at one of the casino hotels on the Vegas Strip.

In early August, Paddock booked a room at Chicago's 21-storey Blackstone Hotel that overlooked the park where the Lollapalooza alternative music festival was being held, though there's no evidence he actually stayed there, a law enforcement official said on Thursday.

The official was not authorised to discuss the case publicly and spoke on condition of anonymity after being briefed on the investigation.

The hotel confirmed a Stephen Paddock made a reservation but said he never checked in.

Lollapalooza draws hundreds of thousands of music fans every year to Grant Park.

Las Vegas shooting – in pictures Show all 15 1 /15 Las Vegas shooting – in pictures Las Vegas shooting – in pictures People scramble for shelter at the Route 91 Harvest country music festival after gun fire was heard Getty Las Vegas shooting – in pictures People carry a person at the Route 91 Harvest country music festival after shots were fired David Becker/Getty Las Vegas shooting – in pictures People run from the Route 91 Harvest country music festival after gun fire was heard David Becker/Getty Las Vegas shooting – in pictures A handout photo released via Twitter by Eiki Hrafnsson (@EirikurH) showing concertgoers running away from the scene (C) after shots range out at the Route 91 Harvest festival on Las Vegas Boulevard EPA/Eiki Hrafnsson Las Vegas shooting – in pictures People lie on the ground at the Route 91 Harvest country music festival after hearing gun fire Getty Las Vegas shooting – in pictures A man in a wheelchair is taken away from the Route 91 Harvest country music festival after hearing gun fire David Becker/Getty Las Vegas shooting – in pictures People stand on the street outside the Mandalay Bay hotel near the scene of the Route 91 Harvest festival on Las Vegas Boulevard EPA/Paul Buck Las Vegas shooting – in pictures FBI agents confer in front of the Tropicana hotel-casino after a mass shooting during a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Las Vegas police run by a banner on the fence at the Route 91 Harvest country music festival grounds after shots were fired David Becker/Getty Las Vegas shooting – in pictures An injured person is tended to in the intersection of Tropicana Ave. and Las Vegas Boulevard after a mass shooting at a country music festival Ethan Miller/Getty Las Vegas shooting – in pictures Metro Police officers pass by the front of the Tropicana hotel-casino after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures A woman sits on a curb at the scene of a shooting outside of a music festival along the Las Vegas Strip AP/John Locher Las Vegas shooting – in pictures A cowboy hat lays in the street after shots were fired near a country music festival in Las Vegas Getty Las Vegas shooting – in pictures Las Vegas Metro Police and medical workers stage in the intersection of Tropicana Avenue and Las Vegas Boulevard South after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Sheriff Joe Lombardo (2-R) speaking during a press briefing in the aftermath of the active shooter incident on Las Vegas Boulevard EPA

Although Paddock killed himself as a SWAT team closed in, the sheriff said it appeared he had planned to survive and had an escape plan. Lombardo would not elaborate on the plan.

The coroner's office in Las Vegas would not release details of its autopsy on Paddock. Some behavioural experts have wondered whether he suffered from some kind of brain abnormality or had a terminal illness that prompted him to lash out.