WASHINGTON — Cpl. Aaron C. Masa became fast friends with a fellow Marine during field training in North Carolina. But behind his buddy’s back, Masa was sexually abusing his friend’s 3-year-old stepdaughter. He also took sexually explicit photos of the girl and the Marine’s infant daughter.

A military judge convicted Masa last year of sexual abuse of a child and production of child pornography, according to court records and other documents detailing the case. Under the terms of a pretrial agreement, he pleaded guilty and received 30 years in prison.

In total, incidents involving sexual assault in which the children of service members are victims occur hundreds of times each year, data the Defense Department provided exclusively to The Associated Press show. The abuse is committed most often by male enlisted troops, followed by family members.