LAS VEGAS — The abandoned baby strollers, shoes, phones, backpacks and purses strewn for days across the huge crime scene of the Las Vegas massacre were slowly being returned to their owners Sunday to become sad souvenirs of a horrific night.

One week ago, the same scene was home to a happy day of country music for 22,000 people at the Route 91 Harvest festival. A few hours later, when 64-year-old Stephen Paddock opened fire on the crowd from the nearby Mandalay Bay hotel, killing 58 people and wounding hundreds, those thousands were left fleeing for their lives, with no care for the possessions they are now recollecting.

Federal agents have spent the week collecting evidence amid the thousands of items, some of them stained with blood.

“Whatever was dropped when people started running, those items we’re collecting and we’re going to provide back,” Paul Flood, unit chief in the FBI’s victim services division said at a news conference.

The items have been catalogued with detailed descriptions, and some have been cleaned of things including blood. They are now being returned to people at a Family Assistance Center at the Las Vegas Convention Center, starting with a few sections of the concert scene and expanding to others at a time to be announced later.

“Just in general, the sheer size of the space, the amount of personal items that were left there, it’s just a huge undertaking,” Flood said.

Country singer Jason Aldean, who was on stage when the shooting broke out, made a special appearance to open the telecast of “Saturday Night Live” with a performance of “I Won’t Back Down” by Tom Petty, who died in Los Angeles on Monday.

“This week we witnessed one of the worst tragedies in American history,” Aldean said before starting the song. “So many people are hurting. You can be sure that we are going to walk through these tough times together every step of the way.”

Las Vegas hotel and gambling magnate Steve Wynn, who owns casinos that Paddock gambled in but not the Mandalay Bay, said Sunday that his hotels have undertaken special security measures in recent years to identify potentially dangerous guests. Those measures include using magnetometers to detect significant amounts of metal and training housekeeping staff to report suspicious actions like a do-not-disturb sign remaining on a door for extended periods.