A painstaking investigation is being carried out by federal agencies assisting Las Vegas law enforcement piecing together a puzzle left by a sniper responsible for the mass shooting at the Route 91 Harvest music festival.

The FBI investigates atop of the Route 91 Harvest main stage at the Las Vegas Village festival grounds, Wednesday, Oct. 11, 2017. Richard Brian Las Vegas Review-Journal @vegasphotograph

Clark County Sheriff Joe Lombardo, left, with Aaron C. Rouse, special agent in charge for the FBI in Nevada, discusses the Route 91 Harvest Festival mass shooting at the Metropolitan Police Department headquarters in Las Vegas, Monday, Oct. 9, 2017. Erik Verduzco/Las Vegas Review-Journal

The FBI continues to investigate the scene on the 32nd-floor of the Mandalay Bay hotel-casino on Wednesday, Oct. 4, 2017, in Las Vegas. Richard Brian Las Vegas Review-Journal @vegasphotograph

WASHINGTON — A painstaking investigation is being carried out by federal agencies assisting Las Vegas law enforcement officials piecing together a puzzle left by the gunman responsible for the mass shooting at the Route 91 Harvest music festival.

The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives are assisting local investigators who must determine whether the killer acted alone, or whether he was aided in the shooting that killed 58 people and wounded hundreds.

Historically, the FBI and ATF offer assistance tracking the guns, gathering evidence and running information through databases — resources that local jurisdictions often don’t have, said Matt Miller, a Justice Department spokesman for former Attorney General Eric Holder.

“The local authorities are in the lead of the investigation,” Miller said.

But federal agencies are necessary to augment small state and city departments.

“When you have a mass shooting scene like Las Vegas, you are talking about a lot of manpower,” said Dana Ridenour, a retired FBI agent who was part of its Evidence Response Team that traveled to New York following the 9/11 attacks.

“You have to bag and collect all that evidence,” Ridenour said.

The Las Vegas tragedy is classified as a mass shooting and not a terrorist attack. The investigation is being led by the Metropolitan Police Department with assistance from federal and state agencies.

If the investigation found terrorist ties, the federal government would assume the lead in the probe, Miller said, noting that the federal government would be better equipped to conduct an international or national investigation.

However, the high-profile nature of the crime would make it likely that Attorney General Jeff Sessions would be briefed on the ongoing probe and developments of the investigation, Miller said.

The FBI’s role

The FBI was on the scene on the night of the shooting.

“The FBI is vital in the assistance of this investigation,” Sheriff Joe Lombardo said moments after the Oct. 1 tragedy occurred. “They are providing federal resources available in this investigation.”

The bureau is collecting video and evidence from private sources, and it will analyze the computer and surveillance system that Stephen Paddock used in his Mandalay Bay suite.

That evidence will be sent to the FBI laboratory in Quantico, Virginia, where the FBI academy is located.

Lombardo said the FBI will be housing “all of that critical information.”

The gunman used his 32nd-floor perch to open fire on people attending the country music festival below. Police found 23 rifles in his room, some equipped with “bump stock” accessories to make the semi-automatic rifles shoot nearly as fast as automatic weapons.

The immediate investigation afterward continued with search warrants executed on homes in Mesquite and Reno, where additional weapons and evidence were collected.

The investigation is expected to take months, if not a year, to complete, Ridenour said.

After 9/11, Ridenour was on an evidence collection team that sifted through debris to find pieces of evidence after the World Trade Center was hit by commercial jet aircraft and collapsed.

Collecting evidence

At the Las Vegas site, she said hundreds of people would work the scene. An FBI evidence response team would package evidence to be sent to the lab in Quantico.

Gun purchases, the surveillance system the gunman used and evidence gathered at the shooting site would be examined “to make sure another gun wasn’t used, another shooter wasn’t involved,” and determine whether someone else did not help him build the electronic gear.

“Did he buy it, could he build it himself?” Ridenour said of questions investigators would ask.

ATF disclosed that Paddock purchased weapons in California, Nevada, Utah and Texas.

The FBI has spoken with his girlfriend, Marilou Danley, who was out of the country at the time of the shooting. Miller said it appeared from initial reports that Danley was more a witness than an accomplice.

Ridenour said investigators will still want to know if she knew anything of his plans to carry out the attack.

The evidence collected will also help local authorities establish a timeline and perhaps help them understand what drove Paddock to carry out the mass shooting that stunned the nation.

“The fact that surprised me is he had very little criminal history,” Ridenour said. “There were no clues he was going to go off his rocker and kill 58 people.”

Contact Gary Martin at 202-662-7390 or gmartin@reviewjournal.com. Follow @garymartindc on Twitter.