Closing arguments are planned Friday in the trial of a U.S. Air Force base instructor facing the most serious charges in a widespread sex abuse scandal.

Staff Sgt. Luis Walker is charged with 28 counts including rape and aggravated sexual assault involving at least 10 women in basic training at the Texas base where all of the country's airmen are trained.

Walker is among 12 of the base's instructors being investigated for sexual misconduct toward at least 31 female trainees starting in 2009.

Six instructors have been charged on counts ranging from rape to adultery, and Walker is the first to stand trial. Prosecutors call his trial a cornerstone of the larger investigation.

Approximately 35,000 airmen graduate every year from the Lackland base. About one in five is female, and about 90 percent of instructors are men.

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