Click here for a slideshow from the scene of the explosion, or mouse over the Eyewitness News Story Window above to watch video for this story.
A series of initial explosions were the result of the combustion of high- voltage cable.
Investigators say that lead around the cable deteriorated and water penetrated it, causing the eventual combustion and admitting the gases.
The explosion caused major damage to the corner of the credit union building and it claimed the life of veteran firefighter Brent A. Lovrien.
The explosion that killed firefighter Brent Lovrien and injured his colleague Anthony Guzman was the result of "a confluence of events which have not happened in this city in nearly 20 years," said Mayor Villaraigosa.
"As mayor of the city, I'm solemnly committed to you that we'll continue to ask the tough questions and report them to you in the most transparent way possible -- committed to you that we'll take all necessary steps to upgrade our power system and protect our neighborhood from incidents like this in the future," the mayor said.
He was joined by fire Chief Doug Barry, who said Guzman, an 18-year veteran of the Los Angeles Fire Department, had been upgraded from critical to serious condition at UCLA Medical Center in Westwood.
The Fire Department was called to the 8800 block of Sepulveda Blvd. shortly before 2 p.m. Wednesday on a report of two manhole covers being blown off.
"What they have found out is that there were a series of explosions that took place up and down Sepulveda Boulevard and possibly leading to the one that occurred here that caused the fatal explosion for our firefighter," said Assistant Chief Bob Franco of the Los Angeles Fire Department.
Firefighters from Fire Station 95 were first to arrive at the scene. When they arrived at the corner of La Tijera and Sepulveda they saw black smoke coming from the rear of the Water and Power Community Credit Union building.
Lovrien and Anthony J. Guzman led the team of firefighters into the building to investigate the smoke, and noticed it was coming from a locked utility room.
Unbeknownst to the firefighters, doors to the room contained "an electrical panel and, because they were locked and secured, firefighter Lovrien used his rotary saw to make entry. During the course of making entry, the explosion did occur," Barry said.
Lovrien was blown back by the blast and debris from the building. Guzman was seriously injured. They were both airlifted to hospitals, but Lovrien did not survive.
Lovrien, 35, died at Centinela Freeman Regional Medical Center Wednesday afternoon from injuries sustained in the explosion. He was a 13-year veteran, and had been working at Fire Station 95 for the last 2 1/2 years.
"An outstanding young man and we were very proud to have that member of the Los Angeles Fire Department," said Captain Armando Hogan of the Los Angeles City Fire Department.
Governor Arnold Schwarzenegger issued a statement Thursday:
"Firefighter Brent Lovrien was a selfless, extraordinary hero who always answered the call of duty to save lives and protect communities. His courageous sacrifice will be honored and remembered forever by the people of California."
Guzman, a father of four, suffered multiple bone fractures and facial trauma in the explosion. The 48-year-old firefighter has been assigned to Fire Station 95 since February 2002, according to the L.A. City Fire Department.
A civilian was also treated for minor injuries after the explosion and was released at the scene.
A credit union employee says Lovrien saved her life.
"I feel their pain, and just know that he was a hero. He was a hero and I thank him very much for what he did," said Janet Obeso.
Lovrien leaves behind his brother and his parents. On Wednesday, the WPCCU and the Los Angeles Firemen's Credit Union established a fund through the Fire Family Foundation to help Lovrien's family. Checks should be made payable to the Fire Family Foundation with "Brent Lovrien Fund" written in the memo section.
The WPCCU Westchester branch remains closed until further notice, but donations will be accepted at the Sunset branch, 1053 W. Sunset Blvd., the Sun Valley branch, 8413 Laurel Canyon Blvd., the Hope Street branch, 111 N. Hope St., the Montebello branch, 2527 W. Via Campo Blvd., and the Northridge branch, 16840 Devonshire St.
The power is still out on the stretch of Sepulveda Blvd. to about 400 customers, many of them businesses and some homes in the area. Northbound Sepulveda is still closed in the area as the investigation into the deadly blast continues.
Eyewitness News reporters Carlos Granda, Melissa MacBride and Wendy Burch contributed to this report.
Click here for a slideshow from the scene of the explosion.