A fire in an approximately 60-year-old high-voltage cable caused a series of blasts and led to the death of a firefighter as he used a saw to cut into doors to an electrical panel, city authorities said Friday.

The incident was labeled "a freak accident" by Mayor Antonio Villaraigosa, but it focused attention on the issue of the aging electrical infrastructure of the nation's second-largest city.

Firefighter Brent A. Lovrien, 35, was killed, Fire Engineer Anthony J. Guzman, 48, was badly injured and a credit union building in the Westchester area was badly damaged in the Wednesday afternoon blasts.

"The initial explosions were the result of a combustion of high-voltage cable," Villaraigosa said. "But the explosion which destroyed the credit union and took the life of Firefighter Lovrien was the result of a confluence of events which have not happened in this city in nearly 20 years, at least the last time someone could remember."

Battalion Chief John Miller, head of the Fire Department's arson and anti-terrorism section, said the events began with a fire in a high-voltage underground cable under a busy corridor near Los Angeles International Airport.

Smoke from the fire built pressure which led to an explosion that blew a manhole cover near an office supply store, Miller said.

The smoke traveled a half-block to the Department of Water and Power Community Credit Union Building where it created a second explosion. As firefighters were investigating the second explosion they received reports of smoke coming from a utility room at the rear of the building.

Guzman handed Lovrien a power saw to try to open the metal door to the utility room. When Lovrien turned on the saw it ignited gases that had built up in the utility room, triggering the blast that killed him, authorities said.

"This is a dreadful sequence of events," said David Nahai, chief executive officer and general manager of the Los Angeles Department of Water and Power.

Nahai said the high-voltage copper cable was in a conduit made of lead that was degraded.

He said the tragedy was a reminder that the city needs to aggressively replace old cables with modern ones that have synthetic coverings.

The explosions forced authorities to shut down two blocks of Sepulveda Boulevard, a major corridor near the airport. Power also was knocked out to about 400 customers.

Villaraigosa promised that "we'll take all necessary steps to upgrade our power system and protect our neighborhood from incidents like this in the future."

Fire Chief Douglas Barry said Guzman was in serious but stable condition after surgery.