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.
"As it relates to possibly electrical, possibly methane all of those possiblilties still remain," said Captain Armando Hogan of the Los Angeles City Fire Department.
On Thursday night, investigators gathered everyone together that walked in a commercial building where a third explosion occurred, killing one firefighter and injuring another. They were trying to recreate what happened Wednesday afternoon.
A fresh team of investigators from fire, police, and power departments met at 8 a.m. Thursday. Their job is to figure out what caused a series of explosions Wednesday, including the deadly blast that cost a 35-year-old firefighter his life.
"We know an incident of tremendous impact occurred yesterday," said Captain Hogan. "So based on that, we want to make sure it's not an electrical problem, it's not a gas problem. So it's basically a process of elimination."
Investigators know the first explosion happened right before 2 p.m. Wednesday. It blew a manhole cover in the air. It happened again 15 minutes later, and then black smoke started to billow from a commercial building on the street.
Firefighters from Fire Station 95 went into the building for a closer look.
The firefighters were attempting to enter a locked utility room with a power saw when the third blast happened.
"Debris was everywhere and I saw a fireman on a stretcher with blood coming down his face. It was shocking," said Judy Johnson, an eyewitness.
"We saw a lot of equipment out there including the saw, trauma boxes, equipment and so forth. So based on that, all of that is being looked into and it is all still under investigation," said Captain Hogan.
Leading the team was veteran firefighters Anthony J. Guzman and Brent A. Lovrien. 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 10-year veteran of the LAFD, and had been working at Fire Station 95 for the last 2 1/2 years.
At his station flowers have been placed on the front door and the flag is being flown at half-staff. A retired firefighter stopped by Fire Station 95 to pay his respects. He spoke of the pain that firefighters go through.
"I had a son that was killed three years ago. He was my family, my own flesh and blood. These guys are like family to me. They are not flesh and blood, but they are still my sons and my brothers," said John White.
"An outstanding young man and we were very proud to have that member of the Los Angeles Fire Department," said Hogan.
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 and an 18-year veteran of the LAFD, is in serious but stable condition after undergoing surgery. However, doctors at UCLA Medical Center are watching him closely. Guzman suffered multiple bone fractures and facial trauma in the incident. The 48-year-old firefighter has been assigned to Fire Station 95 since February 2002, according to the L.A. City Fire Department.
"We are very concerned about the respiratory aspect and if there are any internal injuries based on the blunt force trauma that he possibly went through," said Hogan.
A civilian was injured as well in the explosion. He was treated and released at the scene.
While investigators work to find out what caused the explosion to rip through the area, people who work along the block are anxious to get back into their buildings. But they are being held back by more than just yellow tape.
"I might be able to get in my building but there's no electricity and there's really nothing we can do without a computer," said Steve Uwagboe, who works nearby.
Debbie Martinez, the manager of the First Federal Bank, was escorted by police back into her building. She said the electricity was still off and the smell was overpowering.
"It's like burning, like if there's something burning, you know, that's how it smells like when there's [an] electrical burn," said Martinez.
As the investigation continues, firefighters are trying to get things back to normal along the street. They re-opened the southbound lanes of Sepulveda Blvd late Thursday.
Los Angeles Mayor Antonio Villaraigosa has ordered that all flags in the city be flown at half-staff in honor of the fallen firefighter.
Eyewitness News reporters Wendy Burch and Carlos Granda contributed to this report.
Click here for a slideshow from the scene of the explosion.