The ensuing minutes and hours, as he ran diagonally across the sprawling lot under gunfire toward his teams, to help manage hundreds of casualties, were both everything he had planned for as an emergency responder and like nothing he had anticipated. “It was an immediate thought of just disbelief,” Mr. Simpson, an advanced emergency medical technician, recalled in a phone interview on Tuesday.

On Sunday night, Mr. Simpson had stationed five ambulances on site and had 16 emergency medical workers at the concert, some of them staffing a white medical tent filled with Band-Aids, water and sunscreen.

When he heard the gunfire, he started running back from the boulevard into the grounds. Mr. Simpson was wearing a polo shirt and cargo pants; he had no bulletproof vest or helmet. Assuming the shots were coming from one or more gunmen on the ground, he dived behind a bar at the southwest edge of the concert site, the area closest to Mandalay Bay Resort and Casino. He did not yet know the gunman was on an upper floor there firing down on the concertgoers.

Mr. Simpson tuned his radio to the channel used for ambulance dispatch. “Shots are being fired,” he remembered telling the Community Ambulance dispatch center. “We’re going to have multiple patients. I need more ambulances now.”

Then he switched the radio back to the channel that his emergency medical teams were using to communicate during the event.