Staff Sergeant Luis Walker, an instructor in the United States Air Force, was sentenced to 20 years in prison on Saturday. The sentencing hearing was the final phase of his court-martial for the rape of a female recruit and the sexual assault of several others, according to the Associated Press. Walker is one of 12 men named in a sweeping scandal involving instructors and their exploitation and assault of women at Lackland Air Force Base in San Antonio, Texas.

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On Friday, a military jury convicted Walker of all 28 counts he was charged with, including rape, aggravated sexual contact and aggravated sexual assault. Prosecutors say that from October 2010 to January of 2011, Walker and his fellow officers assaulted and harassed at least 31 female trainees, with Walker meting out the worst of the abuse.

Survivors of the assaults testified at Walker’s trial, saying that they had been physically forced or coerced by threats against their military careers into kissing, touching or intercourse with the 12 instructors. One testified that Walker lured her into an office and attacked her, forcing himself upon her against her protestations.

Another of the instructors, Staff Sgt. Peter Vega-Maldonado, was placed on 90 days’ confinement when he pleaded guilty in the case in June, admitting that he had sexual relations with ten different trainees. The case of Staff Sgt. Craig LeBlanc, who has been charged with sexual misconduct, obstruction of justice and making false statements, has been referred to a general court-martial.

Lackland graduates about 35,000 airmen and women each year. They’re put through 8 weeks of basic training by 475 instructors, 90 percent of whom are male.