SAN FRANCISCO >> A man was arrested in the murders of five people inside a San Francisco home, as police sorted through a crime scene so gruesome and complex that investigators have yet to determine how the victims died.

"This was a complex crime scene. We had five deceased persons apparently from blunt force trauma. We didn’t know what we had," Police Chief Greg Suhr said Sunday, adding that 40 investigators were working on the case.

Police booked Binh Thai Luc, 35, on five counts of murder. His arrest was a surprise twist in the case that authorities initially said was a possible murder-suicide.

Investigators initially couldn’t determine if a suspect was at large, and they thought at least two of the victims were shot. The chief said an "edged weapon" was involved in the slayings of the three women and two men.

Authorities have not been able to identity all the victims, though the police chief said Luc knew them. Suhr declined to say what led investigators to Luc.

He said Luc had a prior criminal record but did not elaborate. Investigators don’t believe the killings were gang-related, Suhr said.

The bodies were found Friday by a woman with access to the home in the Ingleside District, a quiet middle-class neighborhood in the southern part of the city. The home near San Francisco’s City College is in a thriving immigrant community, largely from Asia.

Also arrested was Luc’s 32-year-old brother, Brian Luc, on unrelated charges of drugs and ammunition possession and violation of probation. Suhr did not provide details of that arrest but said the siblings lived together.

The brothers were both being held without bail Monday.

Prosecutors were still awaiting a report from police before filing charges, though an arraignment could happen as early as Wednesday, said Chief Assistant District Attorney Sharon Woo.

Jail records didn’t show if either man had retained a lawyer.

The woman who discovered the bodies found one man dead in the foyer by the front door, then saw a man and a woman dead in the garage before running out and calling police. When officers arrived, they found two other deceased women.

Investigators believe four of the victims were related.