Staff Sgt. Luis A. Walker -- one of six Air Force training instructors at San Antonio's Lackland Air Force Base charged with raping, adultery or having improper relations with female trainees -- could face life in prison and a dishonorable discharge if found guilty of the charges.
He was arrested 13 months ago.
Staff Sgt. Peter Vega-Maldonado, another of the charged training instructors, was separately sentenced to 90 days in prison, 30 days of hard labor and a demotion in rank and pay after he pleaded guilty to an improper relationship with a trainee.
Vega-Maldonado told prosecutors he had inappropriate sexual relations with 10 women in his training squadron, officials said.
As part of his plea deal, he provided testimony against two other trainers who were charged.
Twelve military training instructors -- the Air Force equivalent of drill instructors -- are under formal investigation, a senior Air Force commander said.
Nine of the 12 are from the same unit, the 331st Training Squadron, Gen. Edward Rice, commander of the Air Education and Training Command, told reporters at the Pentagon June 28.
The Air Force believes the scandal is localized and does not involve all nine training squadrons, though the Air Force is holding a broad investigation, Rice said.
Thirty-five others are removed from their positions pending an investigation, officials said.
Thirty-one women so far are identified by the Air Force as victims in the sex scandal, he said, adding the sexual misconduct occurred over the past 2 1/2 years but the first woman came forward in June 2011 and accused her male instructor of assaulting her.
Lackland is home of an Air Force boot camp that graduates 35,000 airmen annually.
The scandal allegedly goes beyond sex between instructors and students, with some instructors allegedly violating orders to stop talking to and seeing the women they were accused of assaulting, the San Antonio Express-News reported. .
Instructors allegedly pressured the women to have sex, at times by threatening their careers, and then allegedly warned them that their futures would be jeopardized if they told the truth to investigators, the newspaper said.
In a number of cases, the women initially lied to the investigators, the newspaper said.
The Air Force said most of its nearly 500 instructors do their jobs well.
2014: The Year in Music [PHOTOS]