Closing arguments were expected to begin Friday morning in the trial of the Texas Air Force base instructor who faces the most serious charges in a broad military sex abuse scandal.

A seven-member jury of military personnel is hearing the court-martial of Staff Sgt. Luis Walker at Lackland Air Force base. Walker is charged with 28 counts including rape and aggravated sexual assault involving at least 10 women in basic training at the San Antonio base.

Lackland, one of the busiest military training centers in the country, is where every American airman receives basic training. Walker is among 12 of the base's instructors being investigated for sexual misconduct toward at least 31 female trainees starting in 2009.

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

Prosecutors and defense attorneys both rested Thursday, after just three days of testimony.

Prosecutors called 14 witnesses, including one alleged victim who gave a video deposition because she had recently given birth and could not travel to the base.

On Tuesday, one alleged victim fought back tears as she testified that Walker lured her into his office and sexually assaulted her on a bed, ignoring her pleas for him to stop. She and others told jurors they were afraid that reporting Walker's actions would get them kicked out of the Air Force.

The Associated Press typically doesn't identify alleged sexual assault victims.

The defense called just one witness, Technical Sgt. Richard Capestro, who testified that instructors and trainees at the Lackland base are under constant surveillance and officials conduct surprise inspections of trainee dormitory areas without warning — seemingly attempting to cast doubt on the possibility Walker could have committed rape and sexual assault on the premises.

Capestro said there are cameras in the hallways and at least some of the stairwells around base dormitories, and that open microphones allow an official on duty in a control room to push a button and listen in on any activity in the dorms.

Lackland has about 475 instructors for the approximately 35,000 airmen who graduate every year. About one in five is female, pushed through eight weeks of basic training by a group of instructors, 90 percent of whom are men.

Once the case goes to the jury, under Air Force court rules, its members consider every charge and can reach a guilty verdict on each with a simple two-thirds majority vote.

If Walker is found guilty on any charge, sentencing begins immediately and is also decided by the jury.

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IN PICTURES: Women in the military

Copyright 2012 The Associated Press.