Police said in a press conference Sunday evening that Binh Thai Luc was arrested early in the morning on five counts of homicide. Luc has a prior criminal record.
His brother, Brian Luc, was also arrested and charged with possession of narcotics and ammunition and for a probation violation.
The Luc brothers both live in San Francisco.
Earlier Sunday sources told ABC7 that two individuals detained by the police had been identified as being associated with a Vietnamese street gang and that the motive may have been gambling debts.
Police said the victims appeared to have suffered blunt force trauma but that the Medical Examiners office is still working to identify the victims and determine the cause of death.
Forty police investigators have been working on the case since Friday morning, when a 12-year-old girl found her family dead inside a house near City College of San Francisco.
Police stressed that the family was specifically targeted. "I want to continue to reassure the public that this incident was specific to 16 Howth Street," San Francisco Police Department Chief Greg Suhr said.
Police are serving search warrants in San Mateo and San Francisco and said the investigation is ongoing.
-- ABC7's Vic Lee contributed to this report.