The gunman behind the largest mass shooting in modern U.S. history shot Mandalay Bay Hotel security guard Jesus Campos before opening fire on 22,000 concertgoers across the Strip, police said Monday afternoon. They still have no motive in the shooting that left 58 dead and nearly 500 wounded.

The encounter with Campos on the hotel’s 32nd floor happened at 9:59 PM local time, six minutes before a gunman began firing into the crowd, Sheriff Joseph Lombardo revealed at a Monday afternoon news conference. He reminded surprised journalists he had cautioned them that, in their “zest for” quick facts, some facts could change as the investigation progressed. “What we have learned is Campos was encountered by the suspect prior to shooting to the outside world,” he clarified.

It had been thought the gunman stopped shooting at the crowd when distracted by the security guard, who approached his hotel suite.

Police have a “pretty comprehensive timeline prior to the shooting,” Lombardo said, but he declined to say whether the high-stakes video poker player had been gambling immediately prior to embarking on his killing spree.

Lombardo explained they do not want to disclose his actions up to the shooting “because that is part of the investigation.” At this point. Lombardo added, “We do not know” what caused the shooter to stop firing on the crowd at the Route 91 Harvest festival across the Strip.

The murderer had personal protection equipment in his suite on the 32nd floor of the hotel and was drilling in the room, which caught Campos’ attention as he was investigating an alarm related to an open door in another room of the floor, Lombardo told reporters at news conference. It is believed the shooter was drilling to place a camera or a rifle in position in the suite, the sheriff said.

Authorities now think the shooter checked into the suite on September 25, not the 28th as originally believed, though it was unclear if he occupied the room that early.

Authorities have checked out more than 200 instances of the deceased gunman’s activities throughout the Vegas area and found “no evidence to show a second shooter,” Lombardo said. Also, there’s no evidence of an affiliation with a terrorist group or belief in any radical ideology.

But the shooter had personal protection equipment in the room, his car in the garage contained “binary explosives,” and he shot at fuels tanks at the nearby airport, leading authorities to believe he had hoped to escape the hotel by distracting first-responders and sending them to another location.

Lombardo anticipates that authorities will be probing the hotel suite and the concert venue for another week.

Authorities have begun to return personal effects of the concertgoers at the convention center in town. The FBI, Red Cross, and city and county officials are doing an “amazing job” on that front, Lombardo complimented, calling it “not an easy task to undertake.”

“You really don’t understand, until you see rows of personal belongings left behind” by those who fled the scene, the magnitude of the event, Lombardo said poignantly.