Investigators from the robbery-homicide division, along with fire and building officials, are examining the 12,500-square-foot home on the 1500 block of North Viewsite Drive as a possible crime scene, according to police spokesman Richard French.
The Feb. 16 fire claimed the life of 61-year-old Glenn Allen, who died of injuries sustained when the ceiling of the mansion collapsed on him and five other firefighters.
Investigators entered the home Thursday morning to search through the residence one more time. They said they are going through every inch of the property trying to see if there is anything they missed.
"Arson has been ruled out and we are here to assist with that investigation the best that we can and determine if there is anything that could be considered negligence," said Capt. Kevin McClure, LAPD robbery-homicide division.
The cause of the fire is part of the investigation. Inspectors also will try to determine if and when the sprinkler system was inspected and if the home's construction and design played any role in the incident, said David Lara, spokesman for the Department of Building and Safety.
"We're looking at everything related to the cause of the fire and everything that led up to the cause of the fire," said McClure.
Even though firefighters have ruled out arson, the cause of the fire could still be considered negligence against the home owner. Authorities said the investigation could take weeks.
When firefighters responded to the fire, which may have been sparked by a faulty gas fireplace, flames were going through the top of the roof and quickly spread. It burned through a plastic sprinkler pipe, filling the space with water, fire officials have said.
A huge chunk of the ceiling supported by wooden beams finally collapsed and trapped Allen underneath. He died two days later at Cedars-Sinai Medical Center.
The five other firefighters were transported to local hospitals to be treated for lesser injuries.
The homeowners, who had been living there for only about a week, escaped the blaze unharmed before firefighters even arrived. Officials estimated the damage at $250,000.
Allen, a 38-year veteran on the force, was about a year from retirement. He is survived by his wife, Melanie, and his daughter, who gave birth to his first grandson a day after he died.
A large candlelight vigil honoring Allen was held Thursday night at Fire Station 78 in Studio City. The vigil was held at the station at 4041 Whitsett Ave.
His funeral was scheduled for Friday morning at the Cathedral of Our Lady of the Angels in downtown.
Allen was the first Los Angeles Fire Department firefighter to be killed in the line of duty since March 2008, when Brent Lovrien was killed in an explosion near Los Angeles International Airport as he investigated an earlier blast that blew manhole covers into the air.
The Associated Press contributed to this report.