A hotel engineer at the Mandalay Bay Resort and Casino says he radioed for help as soon as gunman Stephen Paddock started shooting at him and a security guard.
"I could feel them (bullets) pass right behind my head," engineer Stephen Schuck told NBC News' "Today" on Wednesday. "Something hit me in the back."
Schuck was on a higher level of the Las Vegas hotel on Oct. 1 when he got a call to look at a fire exit door that wouldn't open on the 32nd floor.
That's the same floor where Paddock would fire on a concert 1,200 feet below, killing at least 58 people.
New details released earlier this week indicate Paddock shot hotel security guard Jesus Campos six minutes before he shot at the crowd — contradicting an earlier statement that he was wounded after the mass shooting began.
Schuck was just entering the hallway when the first round of bullets went off at about 9:59 p.m.
"As soon as they stopped, I saw Jesus pop out….he yelled at me to take cover," Schuck said. "As soon as I started to go to a door to my left, the rounds started coming down the hallway."
He said he tried to think of ways to get to Campos, figuring Paddock would have to stop shooting at one point. But the hail of gunfire was "kind of relentless."
Schuck said he radioed for help once the shooting did stop, ran down the hallway and took cover with Campos.
Audio obtained by NBC News also indicated Schuck alerted his superiors that Paddock was firing some 200 rounds within the hotel hallway.
The investigation continues into why Paddock, a wealthy high-rolling gambler, shot at the crowd, wounding more than 500 people.
His girlfriend, Marilou Danley, has been flagged by the U.S. Transportation Security Administration to be notified if she gets on a plane or crosses a border, ABC News reported Wednesday.
Investigators don't believe Danley knew about or played a role the shooting, which happened during a trip Paddock had sent her on to the Philippines.
The new timeline of events has risen questions about the police response to the deadliest mass shooting in modern U.S. history.
MGM Resorts, which owns the Mandalay Bay, declined to comment to the network on the response because the investigation is ongoing.
On the timeline, however, it told NBC News, "We believe what is currently being expressed may not be accurate."
Schuck also credited Campos with saving his life.
"When the first shooting started I was kind of frozen for a second," Shuck told "Today." "If he yelled a second too late, I would have been shot."
At some point Schuck's supervisor gave him a master key to shut off the hotel's elevators. He returned to the 32nd floor to give the key back to his boss, who gave it to the cops so they could enter the room.
Schuck said he plans to return to work, despite the trauma of the situation.
"It's definitely difficult," he told "Today." "But for someone to do something so cowardly and despicable, I'm not going to let that change my life."
"I'm not going to let that define me."