The smoke from a fire in the attic of his family's Blackhawk home killed 19-year-old Benjamin Litman Tuesday night, the Contra Costa County Coroner said in an autopsy report.

The autopsy, completed Thursday, said the teen died from "asphyxia due to smoke inhalation due to a house fire," said Contra Costa County Sheriff spokesman Jimmy Lee.

Fire inspectors believe Benjamin's father, Dr. Robert Litman, who was air-lifted to a hospital in Santa Clara for smoke-related injuries, and his son likely did not have time to escape because the fire started in the attic and didn't trigger any alarms in time, according to a press release issued by the fire district Thursday morning.

It is unclear how long the fire was burning in the attic, said San Ramon Valley Fire Protection District Chief Richard Price.

"When it finally broke through to the bottom floor it was already burning for a while," said Price. "It could have been burning for hours in that attic or it could have been burning for half an hour."

Fire district investigators found that the blaze likely grew undetected by the home's alarm system on the ground floor until flames broke through the second floor ceilings, hurling debris into the rooms below.

When the smoke detector in the house went off, the fire was already freely burning at an advanced stage.

Theresa Patterson, a longtime friend of the Litmans who lives nearby, said she never smelled any smoke or heard any commotion coming from the burning home the evening of the fire.

"It's embarrassing to say I slept through the whole thing," she said. "It's a tragedy.  All the neighbors are beside themselves."

Price said attic fires are uncommon, so smoke dectectors aren't installed there.

"It is a very uncommon place for a fire to start because there is very little up there to start a fire," said Price."It would be very uncommon to have a smoke detector in the attic. You are subject to false alarms with stuff like spiders, dust and vents in the attic."

Price said it is not yet known if the home had other smoke detectors in addition to the main alarm system on the ground floor.

"The big message for us is that smoke detectors should be in the bedrooms and hallways on every floor," he said. "That could have made a difference here."

Investigators are still trying to determine what started the fire in the attic.

Firefighters found Benjamin, a senior at Venture School in San Ramon, dead in his bedroom. His father, a San Ramon-based physician, was found unconscious and remains hospitalized in Santa Jose in critical condition.

The two family dogs were also killed in the fire.

The flames swept through the more than 5,000-square-foot two-story home at 2978 Deer Meadow Drive in Blackhawk, and fire crews responded at 11:15 p.m. Tuesday.

The San Ramon Valley Fire Protection District responded to the call from a fire alarm monitoring company reporting a smoke detector activation. When fire crews arrived they found the roof of the home covered with flames.

Crews responded with six engines, two trucks, two paramedic ambulances, four chief officers, one safety officer and two fire investigators.

The home sustained approximately $2 million in damages and lost contents.