LOS ANGELES -- A Los Angeles police officer has been charged with sexually assaulting a 15-year-old cadet, prosecutors announced Friday.

The Los Angeles County district attorney's office said 31-year-old Robert Cain is expected to be arraigned Friday on charges that include unlawful sexual intercourse.

Cain was arrested in June amid a widening investigation into wrongdoing in the department's cadet program for minors who are aspiring officers.

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Prosecutors said Cain befriended the teenage cadet and had sex with her on three occasions at three locations. Cain's attorney declined to comment Friday on the allegations.

LAPD Chief Charlie Beck stands next to an image of officer Robert Cain during a news conference in Los Angeles on June 22, 2017. Jae C. Hong /AP

LAPD Chief Charlie Beck personally arrested the 10-year veteran last month, CBS Los Angeles reports.

"I find the actions of Cain -- if they are proven -- to be despicable. I find them to be absolutely inconsistent with the ethics and standard of the Los Angeles Police Department and they are criminal," Beck told reporters shortly after the arrest.

In a separate case, Cain has pleaded not guilty to 10 felonies after police said they found illegally modified assault rifles and other restricted firearms while searching his San Bernardino County home during the sex investigation.

Neighbors were stunned at the police presence during the arrest, and said, at the time, they were shocked at the charges.

"I'm very, very sad," family friend Myrna Wong said. "He's a good person. He really is."

"I really don't know. I just can't believe that something like that could happen to him. I cannot believe this," Wong said. "It's so incredible what happened."