Richmond , California ( CNN ) -- Investigators say as many as 20 people were involved in or stood and watched the gang rape of a 15-year-old girl outside a California high school 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 what authorities describe as a 2½-hour assault on the Richmond High School campus in suburban San Francisco . Two teenage suspects have been jailed , but more arrests , as many as 20 total , are expected , according to a police detective . `` We will be making arrests continually as we develop probable cause , '' said Richmond Police Lt. Mark Gagan . `` 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 under . `` 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 do n'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 were n'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 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 , '' Richmond Police Lt. Mark 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 . A third teenager was being interviewed , Gagan said . `` 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 .
10 people involved in assault , 19 others watched and offered no help , police say . California , Richmond , police say student was gang raped for midnight . Former student , 10 , and 15-year-old arrested . Victim , 15 , remains in the hospital in stable condition .