Soccer Coach Sexually Assaulted Teen While Killing Him, Prosecutors Allege

The soccer coach charged with the murder of a California teenager also sexually assaulted the boy, according to prosecutors.

TMZ has learned the L.A. County D.A.'s Office now alleges Mario Edgardo Garcia-Aquino killed 13-year-old Oscar Omar Hernandez while engaged in the "performance of a lewd and lascivious act."

Oscar's body was found last week in a ditch in Oxnard, CA ... and his family said his coach, Garcia-Aquino, was the last person seen with him.

Police initially arrested Garcia-Aquino last week in connection with an unrelated sexual assault -- but, on Monday they said they had enough evidence to charge him with Oscar's murder.

As for the other alleged victim, who prosecutors are only identifying as Julian A., Garcia-Aquino is now facing 2 charges -- lewd act upon a child age 14 or 15, and assault with intent to commit a felony.