(CNN) -- California police investigating the alleged rape of a 15-year-old girl have arrested five people, a police spokesman said Wednesday.Three juveniles and two adults are in custody in connection with the assault, said the Richmond police spokesman, Lt. Mark Gagan. Authorities have described the incident as a 2Å“-hour assault at the Richmond High School campus in suburban Oakland.
Gagan said police arrested three of the suspects Tuesday night, and he said police expect to make further arrests.
Investigators say as many as 20 people were involved in or stood and watched the crime outside the high school's homecoming dance Saturday night.
Police posted a $20,000 reward Tuesday for anyone who comes to them with information that helps arrest and convict those involved in the alleged assault.
"We will be making arrests continually as we develop probable cause," Gagan said. "With this number of people implicated in the incident, we're going to be making arrests on an ongoing basis."
As many as 10 people were involved in the assault in a dimly lighted back alley at the school, while another 10 people watched without calling 911 to report it, police said.
A 1999 California law makes it illegal not to report a witnessed crime against a child, but the law applies only to children 14 and younger.
"We do not have the ability to arrest people who witnessed the crime and did nothing," Gagan said. "The law can be very rigid. We don't have the authority to make an arrest."
Charles Ramsey, a member of the Richmond school board, said the school district bears some responsibility for the attack. School administrators and police apparently weren't watching the area as they should have, Ramsey said.
The school said it would hold a safety meeting for parents and students Wednesday evening to address the alleged assault.
The victim was found unconscious under a bench shortly before midnight Saturday, after police received a call from someone in the area who had overheard people at the assault scene "reminiscing about the incident," Gagan said.
The girl was flown by helicopter to a hospital, where she was admitted in critical condition. She was in stable condition Tuesday, police said.
Investigators canvassed the community with fliers, which included the reward offer, hoping to identify more suspects Tuesday.
"There is one individual in custody who has made some spontaneous statements that have led me to believe that he is culpable for what happened," Richmond police Lt. Johan Simon said.
Nineteen-year-old Manuel Ortega, described as a former student at the school, was arrested soon after he fled the scene and will face charges of rape, robbery and kidnapping, police said.
A 15-year-old was later arrested and charged with one count of felony sexual assault. Other arrests followed.
"Based on witness statements and suspect statements, and also physical evidence, we know that she was raped by at least four suspects committing multiple sex acts," Gagan said.
"As people announced over time that this was going on, more people came to see, and some actually participated," Gagan said.
The attack occurred on school grounds as the annual homecoming dance was under way inside the school Saturday night, authorities said.