Four teens arrested in the alleged vicious gang rape and beating of a 15-year-old girl outside her high school homecoming dance could spend the rest of their lives in prison — and police expect to catch more suspects.

The four — ages 15, 16, 17 and 19 — were charged Wednesday with rape and enhancements that they acted in concert, which could make them eligible for life in prison.

Contra Costa County Deputy District Attorney Dara Cashman said the three juveniles, whose names she hasn't released, have been charged as adults.

"These are people who played a significant role in the incident," Richmond Police Lt. Mark Gagan said. "I'm confident that more arrests will be made."

Besides rape, the 19-year-old, Manuel Ortega of Richmond, was charged with robbery and assault causing great bodily injury. It was unknown if he had an attorney.

The other three face one count each of felony rape with a foreign object. They were charged as adults because of the severity of the crime, Gagan said. The 16-year-old also faces robbery charges.

A fifth suspect arrested Tuesday, 21-year-old Salvador Rodriguez of Richmond, was jailed along with the other four Wednesday but has not been charged.

"We believe there are three or four more we can arrest," Gagan told Fox News on Thursday.

More coverage at KTVU.com.

The alleged gang rape and beating Saturday night at Richmond High School have rattled the city of about 120,000 in the San Francisco Bay area.

Police believe as many as 10 people ranging in age from 15 to mid-20s attacked the girl for more than two hours in a dimly lit area. As many as two dozen people witnessed the rape without notifying police.

Cashman, head of the Contra Costa District Attorney's Office sex crimes unit, told the Contra Costa Times, a Bay area newspaper, that those who witnessed the alleged rape and did not report it could face aiding and abetting charges — if it can be proven that their actions facilitated or goaded the perpetrators.

But under California law, witnesses to the gang rape cannot be charged with a crime if they idly watched it take place.

Police hope a $20,000 reward will bring more people forward with information.

Police received a tip about a possible assault on campus from a former student, who heard two males bragging about it. Officers found the girl semiconscious and naked from the waist down near a picnic table.

Margarita Vargas, who was watching television Saturday night with others at her home two blocks from the school campus, told the newspaper she reported the assault as soon as she heard about it. The newspaper did not say whether Vargas was a student at the school.

"They think it's cool," Vargas said of the alleged perpetrators. "They weren't raised to respect girls."

Gagan said the victim, a sophomore, 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.

She was released from the hospital Wednesday.

Four Richmond police officers and several school officials were handling security at the dance but were unaware that the attack was happening, according to West Contra Costa Unified School District spokesman Marin Trujillo.

But a friend of the victim's criticized security efforts at a school board meeting Wednesday night, KTVU.com reported.

“At the dance, there were four officers — none of them patrolling the area,” she said. “I looked outside of the gym and I saw 12 to 15 guys sitting there with no (student) IDs. The officers did not check the IDs of those men sitting outside of our campus.”

The Associated Press contributed to this report.