Christopher Fox told a federal judge that when he met a 12-year-old girl online, they immediately began exchanging nude photographs of themselves. When the girl wanted to stop, Fox said he blackmailed her into sending more.

Fox, 21, a former military police officer at Schofield Barracks, pleaded guilty Tuesday to producing child pornography. He faces a mandatory prison term of at least 15 and up to 30 years at sentencing in July. Fox has been in custody with no opportunity for release on bail or bond since his arrest in August.

In exchange for Fox’s guilty plea, federal prosecutors promise to drop possession and distribution of child pornography, sending obscene material to a minor and cyberstalking charges.

Assistant U.S. Attorney Morgan Early told U.S. District Chief Judge J. Michael Seabright that the girl was 12 years old when she met Fox on Instagram but told him she was 13. Fox was 19. That was in August 2016. She said the two exchanged sexually explicit photographs on Snapchat.

Early also said the girl told Fox she was in England.

Fox told Seabright he doesn’t remember from where the girl was communicating with him but knows it wasn’t Hawaii. He said he created a fake account and used it to blackmail the girl.

Federal investigators say Fox labeled the Instagram account with the girl’s name and posted nude pictures of her on it. The girl and her parents went to English police after the pictures started circulating at her school.

Fox was a member of the 728th Military Police Battalion at the time of his arrest. The Army discharged him in December.