Richmond, Calif. ? The gang rape and beating of a 15-year-old girl on school grounds after her homecoming dance was horrific enough. But even more shocking, police say, was that up to two dozen people watched and did nothing to stop it.

The attack over the weekend rattled this crime-ridden city of 120,000 in the San Francisco Bay area, where one police official called it one of the most heinous crimes he has ever seen. Some students have already left the school in response to the attack.

“It’s not safe there at all,” said 16-year-old Jennie Steinberg, whose mother let her transfer out of the school Tuesday. “I’m not going back.”

The victim, a sophomore, had left the dance and was drinking alcohol in a school courtyard with a group when she was attacked, police said.

Two suspects were in custody Monday, but police said as many as seven ranging in age from 15 to mid-20s attacked the girl for more than two hours at a dimly lit area near benches Saturday night. As many as two dozen people saw the rape without notifying police.

Officers found the girl semiconscious and naked from the waist down near a picnic table. She remains hospitalized with nonlife-threatening injuries.

“This was a barbaric act. I still cannot get my head around the fact that numerous people either watched, walked away or participated in her assault,” Lt. Mark Gagan said Tuesday. “It’s one of the most disturbing crimes in my 15 years as a police officer.”

Gagan would not comment on rumors that observers took video of the attack on cell phones and may have posted it online.

Manuel Ortega, a 19-year-old former student, was arrested after trying to flee the scene. He is being held on $800,000 bail for investigation of rape and robbery. Attempts to reach Ortega at the jail Tuesday were unsuccessful.

A 15-year-old student also was booked late Monday on one count of sexual assault, Gagan said.

Late Tuesday, SWAT teams were preparing to make more arrests as police are also offering a $20,000 reward they hope will bring more people forward with any information.

Even though he said as many as two dozen people were witnesses, Gagan said officials are still trying to determine the exact number of people involved.

“I’m confident that the list will expand and at the end of our investigation we will get a clear indication of who was there and who did what,” Gagan said.

The attack occurred in a city that has dealt with its share of vicious crimes in recent years, and the school recently approved surveillance cameras after a series of violent crimes. In one case a few years back, a student was shot outside the school, ran inside and died in the then-principal’s hands, said Marin Trujillo, a spokesman for the West Contra Costa Unified School District.

Richmond is an industrialized conclave near the San Francisco Bay that is known as one of the nation’s most dangerous cities. In 2007, Richmond had 47 homicides, and the murder rate led the state for cities with populations of 100,000 or more, surpassing Los Angeles and Oakland.

That number dropped to 27 in 2008 but has spiked to 44 killings so far this year, amid drug dealing and gang activity that has engulfed the town, Gagan said.

Gagan said the girl left the dance and was walking to meet her father for a ride home when a classmate invited her to join a group drinking in the courtyard. The girl had consumed a large amount of alcohol by the time the assault began, police said. Gagan said the girl’s father tried to call her cell phone, but no one answered.

Gagan said police received a tip about a possible assault on campus from a young woman who heard two males bragging about it. She was found nearly an hour after the dance, which more than 400 people attended, had ended.