SAN FRANCISCO -- Three people were injured -- one critically -- in a nightclub shooting early Saturday morning, police said.

The incident apparently started with an argument around 1:15 a.m. inside the 330 Ritch nightclub, located on Ritch Street, an alley between Brannan and Townsend streets, according to San Francisco police officer Carlos Manfredi.

Officials were still working to determine the sequence of events in the shooting, but investigators were examining areas inside and in front of the club as well as a nearby parking lot, Manfredi said.

The victims were all men in their 20s. One man was shot in the torso and was in critical condition when he was hospitalized; Manfredi did not know his condition later Saturday morning.

The other two men suffered injuries that were not life-threatening.

Investigators have "no idea" what caused the fight, Manfredi said.

No suspects were in custody as of about 11 a.m., and no descriptions were available.

Contact Daniel M. Jimenez at djimenez@bayareanewsgroup.com. Follow him at Twitter.com/DMJreports.