"A firefighter died in the line of duty today," Los Angeles Mayor Antonio Villaraigosa said. "It's a sad day in Los Angeles -- clearly a sad and tragic day in Los Angeles."

The death of the firefighter, a 10-year veteran whose name was withheld until relatives could be notified, was the first involving a Los Angeles firefighter in four years.

The second firefighter's injuries were serious, the newspaper reported.

The fatal explosion occurred in an above-ground electrical vault in the Water and Power Community Credit Union about a half-hour after an underground blast tossed two manhole covers into the air on Sepulveda Boulevard, the Long Beach Press-Telegram reported.

The firefighters were struck by the second blast while checking to see if the first one had started a fire.

The second blast was reportedly much stronger than the first and blew a large hole in the back wall of the credit union.

Los Angeles Deputy Fire Chief Mario Rueda told the Press-Telegram the cause of the explosions was not immediately known.