Capt. Joe Geeb, 43, of the Clark County Fire Department is shown Thursday at the Clark County Fire Department Training Center in Las Vegas. Geeb was one of the first responders to the shooting scene and worked with police to use a new lifesaving tactic. (Salwan Georges/The Washington Post)

Joe Geeb didn't know if there was one shooter, or 30.

When the call for a “mass casualty incident” blasted through the radio Sunday night, the Clark County fire captain had no idea what was happening on the Las Vegas Strip, but he immediately began thinking about how he would run toward the bullets, the mayhem and the carnage while everyone else was running away.

He quickly donned his flak vest and the helmet designed to withstand rifle fire and gunshots. Then he paused as a group of armed police officers created a protective bubble around him and other firefighters. Moving as one, the team hurled itself into the center of the chaos.

“I knew the officers had my back, and I would have had theirs,” Geeb said. “We’re going to go in together, and we’re going to come out together.”

Relationships between the nation’s police and fire departments can range from friendly rivalries to downright acrimony. In Las Vegas, officials are confident that an innovative effort requiring both agencies to train together to respond to active-shooter incidents saved countless lives in the massacre that left 58 dead.

Fire departments traditionally have waited on the sidelines of shooting scenes until police declare it safe for medics to go in and treat victims. In some cases, including high-profile mass shootings, that resulted in wounded patients bleeding to death even though medics could have saved them with immediate aid.

Learning lessons from the shootings at Columbine High School outside Denver in 1999 and at a movie theater in Aurora, Colo., in 2012, Nevada’s first responders decided they should work while under fire.

“We saw from the reports of how these people died and the lack of interaction with the police departments and we knew we had to fix that,” Clark County Fire Chief Greg Cassell said Thursday.

Cassell said police and fire agencies in Nevada have been working together since 2010 to develop concerted responses to critical incidents, but Sunday was the first time their years of training and drills deploying “rescue task forces” played out in real life.

An agent walks out of Mandalay Bay Resort and Casino, where Stephen Paddock, the gunman who killed 58 people and wounded nearly 500, stayed Sunday night, using his room as an elevated shooting position. (Salwan Georges/The Washington Post)

Sixteen such task forces raced into the concert venue the night gunman Stephen Paddock opened fire from the 32nd floor of the Mandalay Bay Resort and Casino, sending volleys of bullets down on a country music festival with 22,000 attendees, fire officials said.

Each task force included four to six armed police officers, who created a perimeter around three paramedics, said Roy Session, the deputy chief of Clark County fire operations, who deployed the teams throughout the night. The medics treated and transported the wounded to ambulances under the blanket of safety those officers provided, moving in unison with police from patient to patient.

“What we discovered in Columbine and Aurora is that people were laying and dying waiting for help,” Session said. “This team was trying to avoid that.”

The fire department has trained with police on skills including kicking down doors and treating patients in simulated live-fire environments, with blank rounds fired to ensure medics learn to do their lifesaving work while disrupted by the sound of gunfire.

Sgt. Branden Clarkson, of the Las Vegas Metropolitan Police Department, was one of the many officers who stood shoulder-to-shoulder with firefighters at a Clark County fire department news conference Thursday to discuss Sunday’s rescue efforts. A decade ago, officials said, it would have been unusual to have even a single police officer at a fire department event.

“We have a joking relationship and a friendly rivalry, but when it comes down to it, we know we are there for each other,” said Clarkson, who heads the police department’s efforts related to “multi-assault counter-terrorism action capabilities.”

Las Vegas Metropolitan Police Department Sgt. Branden Clarkson, 33. (Salwan Georges/The Washington Post)

Firefighters and medics faced several challenges Sunday as they responded to what they would later learn was the deadliest mass shooting in modern U.S. history, Cassell said.

People with gunshot wounds appeared at various hotels, but the calls were dispatched to first responders as a report of a shooter at each hotel. The calls initially generated more than 30 different possible shootings along the Strip.

“That complicated our response,” Cassell said. “We’re starting to think, ‘Uh-oh, we’re having attacks at other locations.’ ”

People scattered into hotels, to the tarmac of the airport and into neighborhoods, creating a response area that stretched about a mile.

“It was not in one building, it was not in one spot, it was not in one address,” Cassell said. “It was spread over a massive area.”

The fire department transported more than 200 people to area hospitals that night and treated victims suffering from gunshot wounds, fractures and trampling injuries from the stampede to escape the gunfire.

Some of the injured were wounded further as they were transported out. In some cases, those escaping loaded the most-injured person in a group into a truck or car first because the individual could not move. But then other people would pile on top of that person to get out of the area. Geeb and his team at one point helped 10 people who had crammed into a compact car.

“Then there was the emotional chaos,” said Geeb, whose team treated dozens of people on Sunday. He said people around him kept screaming: “‘Where is it coming from? What’s happening? Where do I go?”

No firefighter or medic was struck by gunfire or injured, except one who hurt his knee in a fall.

Session said the response to Sunday’s shooting was built on lessons learned from previous incidents, but training has focused on a shooter who is at ground level, not someone firing from an elevated position.

He said the department is already breaking down its response to Sunday’s shooting to determine how to adjust for the future.

“Our goal is to learn from those lessons,” Session said. “Hopefully, we won’t have to use this again.”

Cassell said that since Sunday’s shooting, law enforcement agencies and emergency officials from all over the world have contacted him to commend him on the department’s response and to ask for advice on how they can emulate it in the case of a similar incident.

Although Sunday’s shooting was nothing for which anyone could have ever planned, Cassell said, it was something police and fire were ready to handle. It all came down to preparation and relationships, Cassell said.

“We love our cops, and they love us,” Cassell said. “That paid off for us the other night”