LOS ANGELES (KABC) -- Nearly 100 weapons, including "modified assault rifles," were found inside the home of an LAPD officer suspected of having a sexual relationship with an underage cadet, a source confirmed to ABC News.Officer Robert Cain, 31, was arrested Thursday amid an ongoing investigation into the theft of police cruisers and other equipment by juveniles in the department's cadet program.Authorities will be trying to figure out if the weapons are legal under California law.LAPD Chief Charlie Beck said he personally arrested Cain , who has been on the force for 10 years. Cain is suspected of having a sexual relationship with a 15-year-old female cadet. She is one of seven cadets who have been arrested so far in the probe.The investigation was sparked after three teen cadets led officers on two separate chases and crashed two stolen vehicles in South Los Angeles on June 15.The two vehicles were recovered after the crashes, and a third vehicle was located near 76th Street and Central Avenue.Detectives also found two Tasers, two radios and a bulletproof vest that one of the teens was wearing when he was arrested.Beck also said there were "a couple of occasions" where the cadets were believed to have made traffic stops in the stolen vehicles.Authorities said the cadets accessed a check-out system using the name of an officer on vacation to take out the vehicles.On Tuesday, four more cadets were arrested in connection to "taking, operating and joyriding" of police vehicles, Beck said.Councilman Mitchel Englander, who is also an LAPD reserve officer and head of the council's public safety committee, is demanding answers."We have thousands of cadets that graduate successfully out of these youth programs each and every year," he said.He submitted a motion Friday for an independent audit of all of the police department's youth programs, which includes the cadet program and police activities league.The LAPD had no comment on the councilman's motion. Englander said he isn't sure who could do the audit, but suggested the city's controller could be a possible candidate.The cadet program is designed to help at-risk youth to stay away from gangs and drugs in the city. Some cadets said they hope the poor choices of a few cadets will not reflect badly on the entire program."I feel from what I've learned mostly is being responsible," cadet Caroline Hernandez said.Englander said he doesn't have a timeline on when he wants the audit done, but wants to make sure the department's internal investigation and the city council's audit are both thorough.The department is conducting a "top-to-bottom review of the program" as well as equipment check-out procedures. Other officers, LAPD members and cadets have been interviewed as part of the investigation.A graduation ceremony for this year's cadets is expected to continue as scheduled on Saturday.