A Navy reservist from Prince William County is expected to plead guilty Friday to child exploitation charges after military investigators found thousands of images of child pornography on his computer, including videos of the 19-year veteran raping young girls, according to court documents.

Lt. Cmdr. Scottie Lee Martinez, a Navy intelligence officer from Dumfries, is scheduled to appear in federal court this morning to admit to coercing two girls, ages 11 and 5, into sex to make child pornography, court records said. Martinez, 37, faces 15 to 30 years in prison.

The investigation began in March on the other side of the globe. Martinez was deployed to Camp As Sayliyah, in Qatar, when a Air Force captain in the psychological operations logged on to the office computer and noticed that someone was using the system to access child pornography. She immediately notified her superiors, who then walked across the office, knocked on Martinez's door and found him on his personal laptop using the same computer line that was just being used to access the pornography, charging documents said.

A search of his computer unearthed thousands of pornographic images and videos of children, according to an arrest affidavit. Some of the images appeared to be of Martinez having sex with young girls, the documents said.

Naval Criminal Investigative Service detectives said the videos were shot in Dumfries and in Iceland, where Martinez had been stationed in 2006.

According to charging documents, Martinez's ex-wife reported to Houston authorities in December that she suspected that Martinez was having sex with children. The ex-wife provided a hard drive with suspected images to law enforcement officials in Texas, but the investigation was closed because authorities told her there was insufficient evidence to file child pornography charges.

Investigators traveled to Oklahoma, where the alleged victims had moved, and one of the young girls told authorities that Martinez began molesting her and taking photos of her when she was 6, the documents said.

Prosecutors said the crimes took place from 2006 to 2009.