"Originally, I thought the phone short-circuited or burned up to get hot enough to cause a fire, but that's not the case," Tweedy said.

Tweedy, who has been a firefighter and fire investigator for more than 20 years, said he has not encountered anything similar. "This is one of those things where I wish I wasn't the investigator responding," he said, alluding to the expected legal battle over the cause of the fire.

"I was confident with my initial findings, but on the first day I had no ability to test the phone," he said. When he got the burned phone, he took out the battery and packaged it separately from the phone to transport to his office in case the phone caught fire in his truck or office, he said.

Tweedy plans to interview Picaso again once his condition improves. Fire officials said he had a blood alcohol level of .325 and was interviewed with the help of a Spanish interpreter shortly after the fire, but Picaso could not say what happened. Damage from the fire was estimated at US$75,000 in a revised estimate. Included in the damage was a plastic patio chair that was nearly incinerated just outside the bathroom where Picaso was found. The chair was directly under a sprinkler, which was activated and helped contain the fire.

Tweedy said his investigation has been reviewed by other members of his department as well as by the Nokia expert and an insurance investigator. The insurance investigator also initially believed the phone was the cause of the fire, he said.