A Golden Gate Park transient faces murder, torture and assault charges in a group attack on a 66-year-old park dweller whose body was found floating in Alvord Lake last month.

NBC Bay Area has learned that the May 24 attack allegedly occurred after the suspect, Stephen Billingsley -- along with other members of a group of transients known as “travelers’’ -- branded the victim, Stephen Williams, as a sex offender.

In a video obtained by NBC Bay Area, men shout at and taunt Williams. According to investigators, Williams was targeted by the group after he was spotted exposing himself in the park. In the video, as other park denizens hurl insults, one man is seen pulling the victim by the arm and dunking him in the lake.

Investigators say Williams was attacked three separate times starting on May 22 until his body was found by a park gardener two days later.

Billingsley, 19, pleaded not guilty Monday to murder, torture, conspiracy, elder abuse and assault in San Francisco Superior Court.

A second defendant, 36-year old Nikki Lee Williams, faces charges in the case as well, including assault, conspiracy, elder abuse and false imprisonment.

Judge Edward Torpoco ordered both men held on $1 million bail. Their attorneys declined to comment on the case.

The victim does not have a history of sex offenses, but sources say they have evidence to support to support the lewd act. Investigators say that Williams has a mental health history and may have either not understood the commands or had no place to go.

Investigators say Billingsley -- known as “Pizza” Steve --walked into Park Station after the attack and confessed to being involved.

The second suspect, Nikki Lee Williams, known as “Evil,” is alleged to have taken part in the attacks but is not charged with murder. Court documents allege the second suspect was involved in the first attack on the victim. They say Nikki Lee Williams pushed the victim into Alford Lake on May 22 and began “striking and dunking him repeatedly.”

The following day, Billingsley, along with “persons unknown,” according to the allegations, “repeatedly stomped and struck” the victim near Haight and Clayton streets, rendering him unconscious.

The final day, May 24, the charges allege, both of the accused joined other unidentified suspects in attacking the victim a third time, rendering him unconscious.

That day, prosecutors say, Billingley and others dragged the victim into the lake, where he was found dead several hours later.

As many as eight or nine men may have been involved in the attacks, authorities say.

Police said that besides the “travelers,” they have identified at least two gangs in the park – that claim parts of the park as territory. They are investigating whether members of those groups may have also been involved.