The fire was ignited at the front of the building, leading to an arson investigation and calls from the Chinese government for better protection of diplomats in the US, the FBI said.
"An incendiary device fuelled by gas was detonated at the consulate," David Johnson, FBI special agent in charge of the San Francisco division, said at a news conference yesterday.
Johnson did not provide any specifics about a possible motive or suspects.
No one was hurt in the fire that charred a doorway, damaged the lobby and burned toward the roof. No bomb-making materials were found, the FBI said earlier.
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Surveillance footage showed a person coming out of a van parked outside with two buckets of gasoline, pouring it on the front of the building and setting it on fire, said Wang Chuan, a spokesman for the consulate.
"We strongly condemn this despicable act and have already made representations with the US on the attack," Wang said.
"And we hope that the US takes all necessary measures to provide adequate protection to the consular personnel and properties and bring the culprits to justice as soon as possible."
Firefighters brought the flames under control within minutes, fire department spokeswoman Mindy Talmadge said.
That fire came on the day San Francisco supervisors heard public comment on China's human rights record months prior to the 2008 Summer Olympics in Beijing.
The FBI said the 2008 incident was not related to Wednesday's fire.
California Assembly Speaker John Perez condemned the blaze.
"This kind of shameful attack has no place in California," he said.