LAS VEGAS — The gunman who killed 58 people and wounded hundreds more at a country music festival here last week shot a hotel security guard before, not after, opening fire on concertgoers hundreds of yards away, the authorities said on Monday, a significant shift in their timeline.

Sheriff Joseph Lombardo of the Las Vegas Metropolitan Police said the gunman, Stephen Paddock, had wounded the security guard, Jesus Campos, around 9:59 p.m. on Oct. 1. Minutes later, Mr. Paddock began raining bullets on the Route 91 Harvest Festival, where thousands of fans were watching the country musician Jason Aldean perform.

In its previous timeline, the police said that Mr. Campos told Las Vegas officers he had been shot at 10:18 p.m., about three minutes after the concert shooting ended. Last week, police leaders praised Mr. Campos as a “hero” who helped evacuate hotel guests at Mandalay Bay Resort and Casino and aided the police in finding the gunman’s room.

Sheriff Lombardo said his department’s initial account was “not completely inaccurate,” and called the adjustments to the timeline “minute changes” that are common in a complex investigation. The sheriff said that Mr. Campos had been on the gunman’s floor to investigate a door alarm in another room and that after being wounded, Mr. Campos immediately told casino security officials he had been shot.

“But what we have learned is that Mr. Campos was encountered by the suspect prior to his shooting to the outside world,” Sheriff Lombardo said at a news conference on Monday, his department’s first update on the investigation in three days.

Sheriff Lombardo also said that Mr. Campos, after being shot, helped protect a maintenance worker from being injured. The sheriff would not say whether he thought the presence of Mr. Campos accelerated or affected Mr. Paddock’s shooting plot.

“I would not make that assumption,” Sheriff Lombardo said.

The president of Mr. Campos’s union did not immediately respond to a request for comment on Monday evening.

The adjusted timeline was released during a news conference that offered no new insights into Mr. Paddock’s motive. Sheriff Lombardo said that he was frustrated with the pace of the investigation, but that federal and local authorities had recently revisited Mr. Paddock’s property in hopes of developing new leads.

Sheriff Lombardo said the police remained uncertain about why Mr. Paddock stopped shooting at the concert site, and did not know exactly when he fatally shot himself.

“This individual purposely hid his actions leading up to this event,” Sheriff Lombardo said. “And it is difficult for us to find the answers to those actions.”

The police continued to refine their account of the shooting on Monday, when Sheriff Lombardo said Mr. Paddock had “attempted to shoot at the fuel tanks” near the concert site. Last week, the police said the fuel tanks had been hit, but perhaps incidentally.

Sheriff Lombardo also said Monday that evidence, including the damaged fuel tanks, suggested Mr. Paddock may have been planning to flee. Explosive materials were also found in his car.

Mr. Paddock might have believed that the chaos caused by the shooting “would have enabled the first responders to be directing their attention in other locations, which would enable Mr. Paddock to just leave the hotel,” Sheriff Lombardo said.

But the sheriff cautioned that the police have little concrete information about the plot. “We do not know whether he had planned to cause additional harm outside of what occurred,” Sheriff Lombardo said.