"They are investigating all possibilities, including copycat behavior," San Francisco Fire Department Lt. Mindy Talmadge said.
Talmadge said this morning's fire was reported at about 3:30 a.m. near the intersection of 21st Avenue and Balboa Street. There were no witnesses to the crime, and the car sustained only minor damage.
Police Sgt. Lyn Tomioka said it appears a book was placed under the car near the driver's-side door and then lit on fire.
Talmadge said today's fire may be a copycat crime and that there has been "some similar malicious juvenile activity in the Richmond District in recent weeks."
Police and fire officials are still investigating more than a dozen other arson cases in which cars were lit on fire in late July, including in parts of the Richmond District.
Tomioka said this is the first car fire in which a book was lit on fire to ignite the vehicle; in other cases different materials were used, including newspapers.
The fires stopped, until today, after the July 30 arrest of a 62-year-old San Francisco woman, Fafa Chan. Chan was arrested in connection with two structure fires in the city in April and July.
Her attorney, Matthew Rosen, said Chan was arrested on suspicion of setting fire to stacks of newspapers outside a building on First Street in the Financial District in April, and outside another building on Otis Street in July. The first fire caused about $500 in damage; the second $5,000.
A judge threw out charges related to the July fire last week, but prosecutors are seeking to re-file that case.
Chan has not been charged for any of the vehicle arsons, though authorities have not ruled her out as a suspect. Her attorney said she has a history of mental health issues and a prior conviction for arson.