The teenager was a police cadet and Cain’s arrest stems from an investigation into police cruisers and other equipment being stolen by minor cadets.
In Beck’s announcement he noted that although Cain had been a part of the force for a decade, he had never been involved with any of the youth programs.
Beck added that search warrants had been issued for the officer’s personal devices, social media accounts and his Rancho Cucamonga home, where moved two years ago with his mother.
Cain was arrested by Beck himself Thursday on a charge of statutory rape.
"I find the actions of Cain, if they are proven, to be despicable," Beck said. "I find them to be absolutely inconsistent with the ethics and standards of the Los Angeles Police Department and they are criminal."
He said that Cain and the cadet had been involved in a "short-term relationship" over the last month, and that their alleged relationship may be connected to her access to department equipment.
Though there were no photos found on the cadet’s phone to suggest a relationship with Cain, Beck said that text messages he’d seen were sufficient to make an arrest.
The alleged victim was one of seven arrests made as a result of police cruisers being taken for joyrides in the days before Cain was taken into custody. Cain was in charge of "the mechanism by which" equipment was checked out of the department, according to Beck.
T.J. Tarjamo, director of the officer’s union, the Los Angeles Police Protective League said, "This is unforgivable and it's a black eye to the LAPD," according to KTLA.
The union issued a statement on Cain’s arrest on Thursday, saying, "There should be zero tolerance for any officer who takes advantage of those they are entrusted to protect. We are deeply disturbed by this development and condemn this officer's alleged actions in the strongest possible terms."