An underground explosion hurled manhole covers into the air Wednesday, killing one firefighter and seriously injuring another as they responded to an initial blast at a building near Los Angeles International Airport, authorities said.

The covers apparently hit the firefighters, one of whom was pronounced dead after being taken to Centinela Freeman Regional Medical Center in Marina Del Rey, Fire Department Chief Douglas Barry said during a news conference.

That firefighter's name was not immediately released because his family has not been notified. He was a 10-year department veteran.

"We are deeply saddened by what happened today, the loss of one of our family members," Barry said. "These types of incidents hit us very, very deeply."

Another firefighter was taken to UCLA Medical Center and was undergoing surgery for a fracture to his ankle and arm, authorities said.

A bystander suffered minor injuries in the explosions that occurred around 2 p.m.

The firefighters saw smoke coming from several manholes that cover an underground electrical vault, Fire Department Deputy Chief Mario Rueda said. Two manhole covers were off when they arrived at the two-story building on a busy commercial strip. They saw dark smoke coming from the back of the building and, when they approached, another explosion blew the covers off, he said.

Fire crews on the scene told the Los Angeles Department of Water and Power that a gas leak may have caused the blasts, the utility's spokesman Joe Ramallo said. But Rueda said there was "no indication, no evidence, that natural gas was involved."

"It appeared to be at least electrical in nature because of the substations that are underneath Sepulveda (Boulevard), but we don't have a cause of the explosion at this point," Rueda said.

"I don't have a theory on how it occurred. I can tell you smoke is combustible and can be explosive when it reaches the right temperature, and with a source of ignition, smoke can be explosive," he added.

Several agencies, including the California Division of Occupational Safety and Health, will investigate, Barry said.

The explosions ripped through the stucco walls on a corner of the building, which houses the Water and Power Community Credit Union.

Witnesses told KABC-TV the blasts were loud, and that some buildings shook.

"I was over here going to the bank and all of a sudden I heard a big boom and then I ducked, I ducked to find which way to run because it was so loud," Oliver Mitchell told the station.

Judy Johnson said she heard two explosions that happened about 10 minutes apart.

"I saw a fireman going down in a stretcher, he had blood going down his face," Johnson said.