Rohnert Park fire officials Tuesday expanded their investigative team looking into the cause of a devastating apartment complex fire, but have been unable to determine a cause.

Rohnert Park Lt. John Marty requested assistance from the federal Bureau of Alcohol Tobacco and Firearms, as well as additional members of Sonoma County's arson and fire investigation team. But, he said so far things are "inconclusive."

"Right now we haven't found anything in particular. We're not focusing on any one thing," he said.

Fire officials initially were calling the Beverly Apartments fire "suspicious" and said the carport area was considered a crime scene because they hadn't yet determined what sparked the fire.

The 1:45 a.m. Monday fire apparently started in a carport and then quickly spread to the old apartment complex on the floors above.

It caused an estimated $1 million in damage, Marty said.

The blaze destroyed six one-bedroom apartments, at least six cars and displaced 15 people.

Several occupants were rescued by a good Samaritan. He caught two young children dropped from the second floor by their mother when the family was trapped by flames.

He also assisted another couple by placing a ladder for them to climb down.

Investigators combed through the charred remains of the carport.

"We're pretty satisfied the fire started in the carport area," Marty said. Investigators need to rule out potential causes such as electrical wiring or barbecues that were used the previous day, he said.

"There was so much destruction, it makes it hard," he said.

They also were looking at whether someone started it on purpose.

The ATF has expertise in car fires, and they have the ability to process laboratory results faster, Marty said.

The six cars in the carport were reduced to steel frames. Two others were damaged severely by radiant heat.

He said signs of what could be accelerant, which would indicate an arson, also could have been fuel from the vehicles.

Marty said there have been a few car fires in the past year in Rohnert Park, "but nothing we term a rash or a pattern."

Investigators could not pinpoint "if they were arson or criminal in nature," he said, and the fires involved solo vehicles on the street, as opposed to numerous cars in a carport.

Authorities said the apartment complex was built about 40 years ago, just before building codes became more specific for fire safety.

The complex had an open attic, cedar shingles that helped the fire spread easily and no ceiling sprinklers, he said.

"It was like tinder," Marty said. "It was a very hot fire."

A large number of surrounding agencies sent firefighters to the two-alarm call.

Investigators will be back again today. Marty said it could take a couple of more weeks to reach a conclusion, whether the blaze was criminal, accidental or undetermined.

"We may never know the cause," he said, adding "the last thing we want to do is classify something as undetermined."

You can reach Staff Writer Randi Rossmann at 521-5412 or randi.rossmann@pressdemocrat.com.

You can reach Staff Writer Clark Mason at 521-5214 or clark.mason@pressdemocrat.com.