Not far from the pulsating music and dancing of the high school homecoming, young men were drinking in a dimly-lit courtyard out of sight of chaperones.
A friend invited a 16-year-old girl to join them, and she started drinking hard liquor, too. Soon another group of young men came over.
The ingredients for tragedy all were present, experts say. A bunch of men. A vulnerable young woman. Alcohol.
What happened next, authorities say, degenerated into a two-hour-long gang rape by as many as 10 males. Another 20 people allegedly watched as the victim was assaulted, beaten bloody and robbed of her jewelry but they did not stop it or call police.
The attack late last month led to six arrests, captured nationwide headlines and put this community of 103,000 on the eastern San Francisco Bay shoreline through spasms of self examination. Hundreds attended support rallies for the victim.
Some saw the crime as an outgrowth of Richmond's street violence and poverty. But experts say gang rapes happen in all segments of society _ white and minority, rich and poor. And they say the attackers often are bonded males ranging from gang members and neighborhood pals to teammates and fraternity brothers.
"Everybody was asking why did this happen?" said Peggy Reeves Sanday, a University of Pennsylvania anthropologist who has written extensively about gang rape. "It's very clear if you look at the male culture and the bonding culture of young males and the adventure and bravado of a social situation."
Authorities said the suspects knew each other from either attending or having ties to the high school. However, they said any apparent bonds quickly eroded during police questioning as the suspects attempted to shift blame to their alleged accomplices.
"Just pointing the fingers at other suspects places them at the scene of the crime," said Steven Clark, a defense attorney and former prosecutor. "I'm sure they're thinking about cooperating with the district attorney if they could get a lesser sentence."
In many gang-rape cases, Sanday said, the victim is drunk, mentally disabled, lured by someone she has a crush on or considers a boyfriend.
The victim here was a Richmond High School student.
After leaving the dance early, she was waiting outside for her father to pick her up when she heard a familiar voice. A classmate invited her to hang out with him and a few guys in the shadowy courtyard.
She hopped a short gate and began drinking with them. More young men joined them.
"That's when the dynamics changed," said Richmond Police Lt. Mark Gagan, noting the assault soon began.
In gang rapes where bystanders are egging on the others, Sanday said, "it is part of the male ritual. It involves proving their sexual prowess."
School officials and authorities said the victim felt betrayed because she knew a few of her alleged attackers and considered one a trusted friend.
"She is a young girl who's impressionable and, I think, wonders if this is what the world is really like," said Richmond police Sgt. Lori Curran, a lead investigator.
The victim has since been flooded with gifts and letters of support. In a letter read by her family pastor during a vigil, she urged the community to remain calm and "let that anger cause change."
As the investigation continues, disturbing questions hang over the tragedy: Why didn't anyone stop the attack or tell the dance's 10 chaperones, four police officers among them?
"Where were our neighbors, our fellow country-people who witness such a horrific crime and are afraid to call 911?" asked Terry O'Neill, president of the National Organization for Women.
In many sexual assaults, the larger the crowd watching, the less likely someone will intervene, said Sharyn Potter, a sociology professor at the University of New Hampshire.
"They worry about retaliation, their social status among their peers, being labeled as a 'snitch' and their own physical safety," said Potter.
It took someone who was nowhere near the dance to alert police.
Margarita Vargas, 18, a former Richmond high student, called 911 from her house, after her brother-in-law heard two guys bragging about that attack. Vargas was honored Tuesday by the Richmond City Council for her "act of humanity."
"I didn't think twice about it," Vargas recently told reporters. "I immediately grabbed the phone because that's something I wouldn't want anybody to go through, or if I were in that situation, I would want someone to do the same for me."
Officers found the victim semiconscious and half-naked, curled up in a fetal position near a picnic bench shortly before midnight. She was hospitalized for several days.
Police have arrested Cody Ray Smith, 15, Ari Abdallah Morales, 16, and Marcelles James Peter, 17, Jose Carlos Montano, 18, Manuel Ortega, 19, and Elvis Torrentes, 21. All face rape and other charges that could lead to life in prison upon conviction. Messages left for their attorneys Tuesday afternoon were not immediately returned.
The defendants have not entered pleas. The juveniles all are charged as adults.
In the aftermath, the school district has installed new courtyard lighting and is purchasing 120 surveillance cameras and improved fencing.
Sgt. Curran said the victim is showing remarkable resiliency, even while suffering vivid flashbacks and awful nightmares. "Quite honestly, she's voiced several times that she just wants to put this behind her," Curran said. "Unfortunately, she can't, because her emotional healing is still in the recovery process."