Sheriff Donny Youngblood said a cause of death for Silva rests on toxicological tests, which could take from 90 to 120 days to be completed.
Youngblood cautioned in an interview Monday that it is too early in the investigation to reach any conclusions about Silva's death.
In an unusual move, sheriff's officials later detained for several hours two witnesses who had videotaped the incident on their phones. They were released only after they surrendered their phones to deputies.
Witnesses to the incident have described a violent scene, in which sheriff's deputies repeatedly struck Silva in the head with batons.
"I saw two sheriff's deputies on top of this guy, just beating him," eyewitness Ruben Ceballos said in an interview Monday. "He was screaming in pain ... asking for help. He was incapable of fighting back ï¿½ he was outnumbered, on the ground. They just beat him up."
One woman frantically called 911, telling the operator: "The guy was laying on the floor and eight sheriffs ran up and started beating him up with sticks. The man is dead laying right here, right now. I got it all on video camera and I'm sending it to the news. These cops have no reason to do this to this man."
The decision to take the witnesses' cellphones has raised further questions about the Kern County sheriff's handling of the incident.
"It makes it look like a cover-up," said David Cohn, a lawyer for Silva's children and parents, adding that he has not been able to see the footage. "What we're all concerned about is, 'Are these videos going to be altered? Are they going to be deleted?' "
Youngblood defended the decision to take custody of the phones as a way of preserving possible evidence. The sheriff said his office obtained a search warrant for the phones.
"We still have to secure the evidence, especially when the evidence can tell us whether we did it right or wrong," Youngblood said.
He said his agency, to remove the appearance of any conflict, has asked the Bakersfield Police Department to analyze the phone videos.
KERO-TV Channel 23 in Bakersfield broadcast a security camera video from the scene showing grainy images of figures pummeling someone on the ground, with about 20 swings of what appear to be batons or sticks. It's difficult to see Silva in the seven-minute video or how many of those swings connected.
Silva stopped breathing and was taken to a hospital, where he was pronounced dead at 12:44 a.m. Wednesday, the sheriff's office said.