Starting early Sunday morning, there were five suspicious fires involving motorcycles and cars in just over a 24-hour period.
The motorcycles were torched Sunday morning in the Haight-Ashbury. In a fire on Downey Street, a parked truck was also damaged.
Then early Monday morning, firefighters responded to a couple of car fires in the Bayview.
Of particular concern to investigators is Monday morning's fire on Commercial Street, a small alley in Chinatown. This one damaged three parked vehicles.
"I can see the fire moving from one car to the other," said Michael Bonnevie.
Bonnevie lives across the street.
"All my roommates were really scared. The fire was a pretty big fire," said Bonnevie.
Multiple sources tell ABC7, arson investigators found evidence that somebody drilled holes in the gas tank of a Toyota Tundra pickup.
"Yeah, they were definitely looking at the gas tank. Someone said that they noticed it was all from underneath the trucks, so it was pretty scary," said Commercial Street resident Jose Benavides.
Arson investigators believe the fire on Downey Street is not related to the Chinatown blaze.
The other fires are still under investigation.
Firefighters Union President John Hanley said this type of arson, in the middle of the city, is unusual.
"We do have a problem, an issue of arson car fires. But usually they're out where nobody can get hurt and we can't get three or four more cars going or neighboring homes going," said Hanley.
Investigators found the owners of two of the vehicles. According to our sources, the third one, the Toyota Tundra with the holes in the gas tank, belongs to an owner in Carmel.
As of Monday afternoon, our sources said investigators were still looking for that person.