LAPD officer accused of unlawful sex with 15-year-old in department's youth cadet program The officer is accused of having a sexual relationship with a 15-year-old.

 -- Los Angeles Police Chief Charlie Beck says he personally arrested one of his own officers on Thursday.

Beck arrested 10-year veteran LAPD officer Robert Cain in what has become the latest scandal involving the department’s youth cadet program. The 31-year-old officer is accused of having a sexual relationship with a 15-year-old female cadet.

The chief, who usually does not make arrests, said, “I told him you’re under arrest for unlawful sex with a minor.”

A cache of nearly 100 weapons was found at Cain's home during a search, an LAPD source confirmed with ABC News Friday. Included in the cache were what the source described as "modified assault rifles."

Investigators are working to determine if the weapons found at Cain's home were legal under California law.

It is unclear if Cain has a lawyer.

The officer is from the same police station, the LAPD’s 77th Street Division, that has been roiled in a scandal involving teen cadets who allegedly stole LAPD patrol SUV’s, crashing two of them in chases last week after they were spotted by on duty officers. The teens are also accused of stealing equipment like police radios and bullet proof vests.

Beck says the teens pulled over drivers and impersonated officers with the stolen equipment but that they did not handcuff anyone.

The 15-year-old girl allegedly involved in the sexual relationship is one of seven cadets who have been arrested for stealing the patrol vehicles. Cain worked in the equipment room where police gear is checked out. Beck says he is not sure yet if Cain helped the teens obtain the stolen items.

When Beck learned of the alleged criminal conduct involving Cain, he said he made the decision to make the arrest himself. “I think it’s important to send a message,” Beck said during a news conference.

“I find the actions of Cain, if they are proven, to be despicable. I find them to be absolutely inconsistent with the ethics and standards of the Los Angeles Police Department and they are criminal. I’m a police officer and I felt it was my duty to make the arrest.”

Beck said the sexual conduct did not happen at the police station and that the relationship was consensual.

The Los Angeles Police union representing LAPD officers is applauding Beck for taking quick action after the department learned about the relationship while investigating the stolen equipment and SUVs.

Los Angeles mayor Eric Garcetti said in a statement, "There is no higher priority than protecting and guiding our young people -— and when they become LAPD Cadets, every parent should feel confident that they will be treated with the utmost care every step of the way."

Garcetti continues, "The charges against this officer are deeply disturbing, and I have been assured he will be prosecuted to the fullest extent of the law. These allegations represent a total breach of the trust we place in everyone who wears the uniform and a violation of the oath all officers take to protect and serve. There is an exhaustive review of the Cadet Program already underway, and I have faith that our program will emerge stronger and justice will be served.”

Last week Beck suspended the youth programs at the 77th Street and Pacific Divisions. Cadets from both stations have been arrested.

Beck says he still supports the program for troubled teens that includes some 2,300 cadets and plans to attend an upcoming graduation ceremony.