Credit: Billy Calzada/San Antonio Express News/AP Photo

The first of 12 U.S. Air Force instructors charged in one of the biggest sex abuse scandals in recent military history was convicted Friday of 28 charges including the rape and sexual assault of several female trainees at Lackland Air Force Base in San Antonio, Texas.

Air Force Staff Sgt. Luis Walker faces up to life in prison and a dishonorable discharge at his sentencing hearing, which begins today.

An ongoing investigation found that since 2009, at least 31 female recruits were alleged victims of sexual misconduct and rape by some of their male drill instructors at Lackland.

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Lackland is the Air Force's largest boot camp training facility, with about 500 instructors - 12 of whom, including Walker, are under investigation. Six of these are facing criminal rape, adultery and aggravated sexual assault charges, including one who pleaded guilty in April to having an inappropriate relationship with a trainee.

Court-martial testimony began Tuesday against Walker, who has the most serious case of the 12 instructors.

Four of Walker's alleged victims took the stand Wednesday, some fighting back tears as they described him luring or forcing recruits into sexual favors, then demanding they not tell anyone.

Col. Polly Kenny, the staff judge advocate, said the Air Force clearly didn't live up to its own standards.

"Our primary goal is safety and security of the trainees and obviously we failed," said Kenny.

Kenny said she hopes the conviction makes a difference for other women who may have been victims.

"I hope the message is to come forward, come forward tell us anything that has happened and the command will investigate it," Kenny added.

ABC News' Julie Percha contributed to this report.