Richmond police have arrested a second suspect in connection with the two-hour gang rape of a semiconscious 15-year-old outside her homecoming dance at Richmond High, authorities said Monday night.

Police arrested and booked a 15-year-old student from Richmond High for felony sexual assault and are continuing to actively search for other suspects — both juveniles and adults — through the night, Detective Ken Greco said. The boy’s name and grade level were not released.

The suspect was one of two students in custody for questioning early Monday evening, Greco said. Authorities have already arrested 19-year-old Manuel Ortega as he ran from the crime scene, police say.

Authorities said people took photos, laughed and some joined in as the girl was repeatedly assaulted. The victim, a student, remained hospitalized Monday with injuries that were not life-threatening.

“She was raped, beaten, robbed and dehumanized by several suspects who were obviously OK enough with it to behave that way in each other’s presence,” said Lt. Mark Gagan, a patrol supervisor in the city’s Northern Policing District. “What makes it even more disturbing is the presence of others. People came by, saw what was happening, and failed to report it.”

Ortega remained at Contra Costa County Jail in Martinez on Monday, booked on suspicion of rape and robbery. Detectives believe as many as six other men raped the girl as she lay semiconscious on a courtyard bench, also beating her, taking pictures and stealing her jewelry.

No one called police until word of the ongoing rape spread to a house party in the city’s North and East neighborhood, where an appalled partygoer felt obligated to phone in the rumor.

“That’s just wrong,” senior class President Gina Saechao, who helped organize the dance, said Monday. “What if it was your little sister? What if it was your mom?”

Officers broke it up, and found the victim semiconscious and obviously hurt. Paramedics flew her to a regional trauma center in critical condition; she stabilized overnight.

This newspaper does not name victims of sexual assault without their consent.

The girl attended the dance, held in the gym from 7 to 11 p.m. Saturday. About 400 students attended, with three administrators and four police officers both inside and watching the surrounding parking lot and street.

“Obviously we’ve had some breakdowns. Obviously, it was not safe because this happened,” said Charles Ramsey, a West Contra Costa school district board member. “Should we have had higher awareness, should we have been more vigilant? Probably.”

Police say the victim left alone about 9:30 p.m. and walked north on 23rd Street, expecting to catch a ride from her father. Instead, a schoolmate caught her attention from behind a cyclone fence on the north end of campus.

He invited her to join a group drinking and hanging out in a secluded courtyard behind the fence, Gagan said, and escorted her down Emeric Avenue to a short gate from which they made their way back to the group.

The assault began after the girl quickly drank a large amount of hard alcohol and fell over, Gagan said.

Detectives spent the rest of the weekend trying to identify the various participants, some of whom arrived after the gang rape began.

The courtyard is pitch-black at night, making it difficult to see into it from 23rd.

“(Lighting) is an ongoing issue for all our sites,” school district spokesman Marin Trujillo said. “That particular section does have lighting. Could it be better? That’s something we’re always reviewing.”

The school district plans to install surveillance cameras by January at the campus, a project long in the works. Plans for new fencing have been in the works since March.

“It’s unfortunate that we weren’t able to have this finalized a little bit sooner,” Ramsey said. “But we’ve been on top of this issue (safety). Our board is working very proactively to make sure we stay on top of the issue.”

Reach Karl Fischer at 510-262-2728 or kfischer@bayareanewsgroup.com. Follow him on Twitter at twitter.com/kfischer510. Reach Shelly Meron at 510-243-3578 or smeron@bayareanewsgroup.com. Follow her on Twitter @shellymeron.