Ten male students at a Los Angeles high school have been arrested in a series of sexual assaults against two underage girls that went on for more than a year and recently intensified, the police said, and officers on Saturday were searching for four more boys who are suspects in the case.

Investigators believe the assaults took place at Venice High School and off campus starting in December 2013 and continued with increasing frequency until this month, according to the Los Angeles Police Department. Some of the crimes were forced sexual acts, and while others were not forced, the victims were too young to give consent, Cmdr. Andrew C. Smith, a spokesman, said at a news conference on Friday.

Eight students were taken into custody Friday morning at the school to protect the victims from further attacks, the police said. A ninth person was arrested off campus, and a 10th surrendered at a police station, according to the police. Local news media reported that at least some of the suspects were members of the school’s football and basketball teams.

“We didn’t want to leave the suspects out there to potentially victimize other girls at the school,” Commander Smith said, according to The Los Angeles Times. “The last thing we wanted was to have another victim.”