An anonymous tip led to the discovery that 10 college students had gathered for a late night hangout at an abandoned property where the fire originated, Santa Barbara County Sheriff Bill Brown said. He declined to say which college the students attended.
"It appears this fire was the result of carelessness, not criminal intent," Brown said.
No criminal charges have been filed, but the county district attorney will review the case, Brown said.
The fire consumed nearly 2,000 acres in Santa Barbara County and critically injured a couple who were burned as they fled their home, which was destroyed by the flames.
The college group left the smoldering bonfire in the middle of the night and the embers sparked the wildfire 13 to 14 hours later, Brown said. The group had been hanging out at a property known locally as the "tea garden" next to an abandoned tea house in the hills of Montecito.
"They thought the fire was extinguished, but we don't have a lot of detail to disclose about what they did to do so," Brown said.
The fire was the first of three blazes to erupt in Southern California in the last week which have collectively damaged or destroyed about 1,000 homes and blackened more than 65 square miles.
In Southern California, the Santa Ana winds that swept six counties like a blowtorch died down Tuesday, allowing crews to mop up the smoldering hotspots.
The fire in the San Fernando Valley was 70 percent contained; a third wildfire in Orange County was 75 percent contained.
For a second day, officials allowed residents to return to the Oakridge Mobile Home Park in Sylmar for a few minutes to salvage what they could from acres of ashes. The fire there burned about 480 homes and left 125 standing.
Residents whose homes were intact were allowed to quickly pick up clothes, toiletries, and other belongings under police escort, reported CBS News correspondent John Blackstone. The scale of destruction was hard for residents to accept.
"It looks like a battlefield," said Rick Asavis. "Just like a bomb went off here."
The county said it was sending crisis counseling teams to comfort the victims.
Help began to come from the state and federal government. President Bush made a disaster declaration for California, freeing federal aid to areas ravaged by the wildfires that blacked more than 65 square miles.
Gov. Arnold Schwarzenegger waived state fees for fire victims who need to replace destroyed birth certificates and other documents or obtain state property inspections. He also waived a one-week waiting period for unemployment insurance applicants who lost their jobs because of the fire.
The state has spent $305 million on emergency firefighting since the start of the fiscal year on July 1, $236 million more than lawmakers had planned.
The firefighting expense is adding to the state's $11.2 billion deficit, the governor's finance spokesman said Tuesday.
The state will try to get some of its costs reimbursed from the federal government, which pays for firefighting on federal land, said H.D. Palmer, spokesman for the state Department of Finance.
Last year's wildfires were the most expensive in state history, costing California more than $518 million.