As hundreds of people on Saturday protested the death of a homeless man during an altercation with Fullerton police, it was looking less likely that a videotape at the center of the case would provide a full answer as to what happened.

Surveillance tape shot from the Fullerton bus depot shows parts of the police altercation, but key elements are obscured, according to a law enforcement source who reviewed it. The source, who spoke to The Times on the condition of anonymity because the case is ongoing, said the tape shows six officers struggling with Kelly Thomas, a 37-year-old man who suffered from schizophrenia.

The source said obstacles obscured the “quality and angle” of the shot. It’s hard to see whether Thomas was restrained, as some witnesses have said, and the full extent of the officers’ actions, the source said. Another source familiar with the video said tree limbs and leaves blocked part of the camera’s view.

Other videos that have been posted on the Internet, one from a bus and another from a bystander’s phone, do not show the police officers but capture the sounds of a stun gun, Thomas’ screams and the reactions of shocked bystanders. Witnesses on those videos describe Thomas being repeatedly struck, kicked and shocked by the officers.

The incident, already under investigation by the FBI and the Orange County district attorney’s office, is roiling local politics as well. Two Fullerton City Council members have called on the police chief to step down. And local conservative activist and entrepreneur Tony Bushala said he had gathered enough signatures to begin the recall petition process against Councilmen Pat McKinley, Don Bankhead and F. Richard Jones.

The July 5 incident, which left Thomas unconscious and in critical condition until he died five days later, has outraged his family and many residents of the Orange County suburb. Some council members have complained that even they cannot get basic facts about what happened, and are calling for the release of the bus depot video.

The man’s father, retired sheriff’s deputy Ron Thomas, has released a photo of his son’s bloody, swollen, barely recognizable face. On Saturday, a crowd of protesters stretched across the intersection of Highland and Commonwealth avenues, chanting, “Justice for Kelly! Jail killer cops!” as passing cars honked in support.

The Police Department has released few details about what happened that night, other than to say that Thomas was stopped by officers investigating a report of an attempted car burglary and became combative.

None of the six officers involved has spoken to the Orange County district attorney’s office. Investigators have interviewed about 80 witnesses and are awaiting test results and a cause-of-death determination from the coroner’s office, according to the district attorney’s chief of staff, Susan Kang Schroeder.

The district attorney has so far declined to make the bus depot video public. Schroeder has said that its release could taint witness testimony.

She also said the tape does not show the full extent of the incident. “It shows certain things but [does] not completely show everything” that happened, she said.

“You understand why the public is upset. We are doing the investigation as quickly as possible,” Schroeder said in an interview last week. “The D.A. has made it very clear: This is a priority. We have two dozen investigators involved in the investigations…. It is such an important case. The public wants the answers as soon as possible, but the public does not want a rush to judgment.” She said the facts will come out either at trial or in a detailed report from her office.

Experts say criminal investigations involving police use of force are not about whether an officer struck the person, but whether the force was “unreasonable” or “excessive.”