The FBI have started returning thousands of purses, phones and other property left behind by concert-goers in Las Vegas fleeing the deadliest mass shooting in modern US history, as the Red Cross stepped up efforts to reach those traumatised by the October 1 massacre.

Investigators still lack a clear reason why Stephen Paddock, 64, unleashed a torrent of gunfire into a crowd of 22,000 at the Route 91 Harvest country music festival. The suspect shot himself to death before police stormed his 32nd-floor suite in the Mandalay Bay Casino and Resort, high above the concert venue.

“We’re past the response portion of this horrible incident,” Clark County Emergency Manager John Steinbeck said at a news conference. “We’re moving into the recovery now.”

Doves are released for each victim of the Route 91 Harvest music festival mass shooting at City Hall plaza in Las Vegas, Nevada, U.S., October 7, 2017. (Photo: Reuters)

Fifty-eight people died and nearly 500 were injured.

Unlike so many other perpetrators of deadly mass shootings before him, Paddock left behind no suicide note, no manifesto, no recordings and no messages on social media pointing to his intent, according to police.

Paddock used a device known as a bump stock to make 12 of his rifles operate more like automatic weapons, which are outlawed in the United States. On Sunday, the powerful US gun lobby, the National Rifle Association, said it would oppose an outright ban on bump-stock devices.

Police officers salute during a memorial service for Charleston Hartfield, an off-duty Las Vegas police officer who was killed during the Route 91 music festival mass shooting, in Las Vegas, Nevada, U.S., October 5, 2017. (Photo: Reuters)

On Sunday (local time), teams of counselors fanned out across the city, attending church services and gathering at a family assistance center set up at the Las Vegas Convention Centre as the Red Cross set out to find those in need of comfort. Spiritual and legal advisers were also available.

“A week into this, a lot of people have been numb,” said Red Cross spokesman Bill Fortune, who flew in from Colorado to help with the recovery effort. “Some of those emotional crises are just showing up today, where people can’t get out of bed. People have called saying they can’t be in crowds.”

The process of returning items left behind by those who fled in the chaos could take weeks, authorities said.

A woman looks at white crosses set up for the victims of the Route 91 Harvest music festival mass shooting in Las Vegas, Nevada, U.S., October 7, 2017. (Photo: Reuters)

So many phones, backpacks, lawn chairs and other items were left behind that the Federal Bureau of Investigation has divided the huge crime scene into four quadrants, releasing items from only one of them at a time, starting Sunday (local time), FBI Victims Services chief Paul Flood said.

Before release, the items had to be cleaned of blood and other substances, as well as categorised, Flood said. Property from just one quadrant of the scene filled seven delivery-sized trucks, he said, and required the attention of dozens of investigators.

Authorities began returning vehicles left at the concert-grounds to their owners last week.

Reuters