The gunman’s first victim in the mass shooting on the Las Vegas Strip wasn’t a festival-goer attending the concert below but a security guard in the hotel where he was staying.

Metro Police said Monday that the suspect, 64-year-old Stephen Paddock, opened fire on a security guard at the Mandalay Bay Resort and Casino who had gone up to the 32nd floor to investigate an alarm set off by an open door. The door in question was not the suspect’s room.

But security guard Jesus Campos — whom police have credited with guiding them to the gunfire’s origin — heard a drilling noise coming from Paddock’s hotel suite and, while investigating, was shot by the suspect six minutes before the shooter opened fire on the crowd at the Route 91 Harvest music festival. Police said the purpose of the drilling was unclear, though it was possible the suspect intended to place a camera or rifle in the hole, which was located near the door to the hotel suite.

Metro Sheriff Joe Lombardo acknowledged the timeline has shifted, noting the complexity of the investigation that has so far continued to baffle investigators.

“Yes, the timeline associated with the original shot at Mr. Campos has changed through investigation,” Lombardo said. “As I have conveyed to you from the very beginning in your zest for information and my zest to ensure the public safety — and the calming of their minds — that some things are going to change.”

Eight days into a far-reaching investigation, authorities don’t seem any closer to pinpointing why the suspect felt compelled to open fire on a crowd of 22,000, killing 58 people and injuring nearly 500 others. Lombardo said they’re speaking to his family members, including a brother who flew into Las Vegas over the weekend, and his girlfriend, Marilou Danley.

Additionally, behavioral analysts are piecing together a portrait of the shooter’s mental state and have revisited his homes in Mesquite and Reno, he said.

“Yes, I’m frustrated,” Lombardo said. “... And it isn't because nobody is not doing their job: It's because this individual purposefully hid his actions leading up to this event, and it is difficult for us to find the answers to those actions.”

He said there is no evidence or intelligence to suggest that the suspect was linked with or in any way affiliated to any known terrorist groups or ideologies, nor is there any new information to suggest a second shooter.

“Currently we do not believe there was one particular event in the suspect’s life for us to key on,” Lombardo said. “We believe he decided to take the lives he did and he had a very purposeful plan that he carried out.”

Police said they have identified 200 instances of the suspect being in Las Vegas. Lombardo declined to detail his locations but said he did not have a companion.

Lombardo also confirmed the suspect fired at jet-fuel tanks near the concert site — as first reported by The Las Vegas Review-Journal — in what may have been an attempt to set off an explosion and distract first responders while making an escape. Investigators found “personal protection equipment” in the hotel room as well as binary explosives in the suspect’s vehicle, which was stowed in a parking garage, the sheriff said.

Authorities have put together a ”pretty comprehensive” timeline of the suspect’s night leading up to the shooting, but they’re not ready to release those details or his medical history, Lombardo said. Police previously said the gunman was gambling in the days before the shooting.

Lombardo, who has largely avoided calling the suspect by name, said it appears he checked into Mandalay Bay on Sept. 25 — three days earlier than originally thought.

“I don’t want to sensationalize his actions,” the sheriff said, explaining his decision to describe him as a “suspect” versus using his name.

As investigators continue combing through evidence, people who attended the music festival can start retrieving personal belongings abandoned at the site. Some items left behind are available at the Family Assistance Center located at the Las Vegas Convention Center.

Evidence processing at the festival site and hotel room likely will take at least another week, Lombardo said. Police don’t anticipate providing further investigation briefings until Friday unless information warrants an announcement sooner.