A sudden explosion was likely responsible for the death Thursday of a 21-year veteran of the San Francisco Fire Department who was battling a two-alarm fire in the city's Diamond Heights neighborhood.

Vincent Perez, 48, entered a burning home at 133 Berkeley Way with other firefighters at about 10:45 a.m., after flames had broken out on the first floor and spread to the second, according to Fire Department spokeswoman Mindy Talmadge.

Everything seemed to be under control, Talmadge said, until some firefighters reported a "flashover" — an explosion caused by superheated material in an oxygen-deficient space. When oxygen is introduced, it creates a blast of heat and light.

“Everything in the room is superheated,” Talmadge said. “Everything just ignites at once.”

The heat alarms on Perez' uniform and that of firefighter Anthony Valerio were activated. Commanders on the outside tried to reach the firefighters by radio, but there was no response. When crews were sent in, Perez and Valerio were found on the ground, "pretty badly burned," according to Talmadge.

They were rushed to San Francisco General Hospital. “We tried everything we possibly could to save his life,” said surgeon Andre Campbell, who led the team that treated Perez. “We tried to support his heart to get him though.”

Perez died at 11:50 a.m. of cardiac arrest. Valerio, 53, is still in the intensive care unit and is "fighting for his life," according to fire Chief Joanne Hayes-White.

A third firefighter, Tracy Courtney, was treated for smoke inhalation and released.

Perez's brother, Lou Perez, a San Francisco police officer, helped carry his body from the hospital to a van for transport to the medical examiner's office. Roughly 40 firefighters stood and saluted as Perez passed.

White, who attended the fire academy with Perez, told reporters of his death in front of the emergency room at San Francisco General Hospital Thursday afternoon.

“It’s a tough day in San Francisco,” she said. “The fire department is like a family, and we lost a family member today.”

Perez, 48, grew up in the Mission District and served in the U.S. Marine Corps. A lieutenant who had worked out of Station 26 in the city's Diamond Heights district for the last 10 years, he came from a family of public safety officers. 

“His whole adult life was in serving and protecting the community,” White said. “We'll never forget Vince Perez.”