The 4 p.m. blaze possibly caused the death of the resident of the home, 51-year-old Alexander Gregory Sujohn, who was found dead inside the residence at about 7:15 p.m.
Coroner's Deputy Jeff Sherman said results of an autopsy today are pending toxicology tests. He said Sujohn was badly burned and he was identified through his dentures.
Whether Sujohn died of his burns or from smoke inhalation is still unknown, Sherman said.
Sujohn was found in the front living/dining room of the small home, Schoenthal said. The natural gas explosion occurred in a portion of the duplex structure that was vacant and had not been rented, Schoenthal said.
Fire investigators are still investigating what caused the explosion itself, he said.
"We haven't been able to pin down what caused it to go off," Schoenthal said.
It is believed Sujohn was the only one living in the residence and he might have been the brother of the owner of the home, Schoenthal said.
The fire caused $150,000 structural damage and $25,000 damage to the contents, Schoenthal said. The fire was contained by 4:30 p.m.
San Rafael police department spokeswoman Margo Rohrbacher said a police officer and several residents of the area heard a loud explosion before the building erupted in flames. The south wall of the house "was completely blown out," Rohrbacher said.
A team of Marin County fire investigators from multiple county agencies is investigating the blaze, as is protocol with fatal fires in San Rafael, Rohrbacher said.