Orange County Dist. Atty. Tony Rackauckas called the footage a key piece of evidence. It has yet to be released to the public.
"Once you view it, you just can't help but feel that we have a tragedy here and... obviously it shouldn't happen," Rackauckas said.
Rackauckas would not elaborate on what the footage shows, except to say the video shows a struggle and that he had not seen any evidence the six officers intended to kill 37-year-old Thomas.
Rackauckas said his office is trying to determine whether officers used excessive force. The investigation is being expedited, he said.
On July 5, officers were investigating reports of someone burglarizing cars at a transit hub and tried to search Thomas' backpack when they got into a fight with him. The police have said Thomas fled and resisted arrest.
Thomas was left badly bruised and swollen in the face. He suffered severe head and neck injuries and was taken off life support July 10.
The six officers were placed on administrative leave.
In a video taken by a bystander with a cell phone, Thomas can be heard screaming out for his father as officers use a Taser on him. In another video from a bus, witnesses describe how officers beat Thomas and how he was left nearly lifeless.
The coroner's office has not determined a cause of death because it is waiting for the results of toxicology tests.
The Associated Press contributed to this report.