LOS ANGELES (AP) _ A firefighter investigating reports of smoke coming from manhole covers was killed Wednesday when an explosion rocked a building near Los Angeles International Airport, authorities said.

The firefighter was pronounced dead after being taken to hospital, Fire Chief Douglas Barry said. He was a 10-year department veteran.

Another firefighter was seriously injured and was being treated for a fracture to his ankle and arm. A bystander suffered minor injuries from the afternoon blast.

"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."

The Fire Department initially said the firefighters were investigating smoke coming from manholes that cover an underground electrical vault and that the covers appeared to have been blown off in an explosion.

But department spokesman Ron Myers said late Wednesday there was "no confirmation" that the covers were blown and that it didn't appear the firefighters were struck by them.

The firefighters saw smoke coming from the back of the building and were trying to enter a utility room connected to the structure when the explosion occurred around 2 p.m., Myers said. The explosion ripped through the stucco walls of the building, right next to that room, and forced authorities to shut down a major commercial strip in the area.

The cause of the blast was under investigation, he said.

The Los Angeles Department of Water and Power said the utility room contained electric and gas meters that were connected to an underground power vault.

"To us, it's not clear where the smoke was coming from and what caused the explosion," utility spokesman Joe Ramallo said. "But if we had an explosion in the underground vault, it could have caused outages and that did not happen."

Ramallo said it was possible a gas leak caused the blast, but Fire Department Deputy Chief Mario Rueda said in an afternoon news conference 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" the street, she said. But the cause was unknown, she added.

Copyright 2008 The Associated Press.