U.S. Air Force Staff Sgt. Luis A. Walker arrives from a lunch break during his court martial at Lackland Air Force Base, Monday, July 16, 2012 in San Antonio. Walker, a former training officer, is charged with illicit sexual contact with 10 female trainees. He is facing 28 counts including rape and is one of 12 instructors under investigation. (AP Photo/The San Antonio Express-News, Jerry Lara) RUMBO DE SAN ANTONIO OUT; NO SALES

SAN ANTONIO - A U.S. military jury Tuesday was to begin hearing the case of an Air Force sergeant who faces the most serious charges in a widening sex scandal at the base where every American airman reports for basic training.

Staff Sgt. Luis Walker is among 12 instructors at Lackland Air Force Base in Texas who are being investigated, and faces the most serious charges — 28 counts, including rape, aggravated sexual contact and multiple counts of aggravated sexual assault. He could get up to life in prison and a dishonourable discharge if convicted.

Six of the 12 instructors under investigation face charges ranging from rape to adultery.

The 10 female recruits Walker is accused of either sexually assaulting or engaging in inappropriate sexual conduct with are expected to testify.

One of the women briefly testified Monday, saying Walker had texted her a photograph of himself shirtless. Defence attorneys had questioned the authenticity of the photo.

Prosecutors say Walker told the woman that she was "hot" and "should have been naked." The Associated Press is not naming her because she is an alleged victim in the sex scandal.

At least 31 female trainees have been identified as victims.

Officials at Lackland call Walker's court-martial the "cornerstone case" in the investigation.

Lackland is where every American airman reports for basic training — about 35,000 a year. About one in five is female, pushed through eight weeks of basic training by a flight of instructors who are about 90 per cent male.

The sexual misconduct at the base apparently began in 2009, but the first woman didn't come forward until last year. The first allegations were levied against Walker, who is accused of crimes that allegedly took place between October 2010 and January 2011.

According to the Air Force charge sheet, Walker had sex with four of the 10 female recruits. He also is accused of forcing five recruits to engage in sexual acts by threatening their military careers and intimidating two of the women into lying about his alleged misconduct, according to the charge sheet.

Walker was a trainer for about 18 months, until he was removed from his position in June 2011. The Air Force is withholding his age and hometown.

The first court-martial in the case resulted in a plea agreement in June, when Staff Sgt. Peter Vega-Maldonado admitted to having sex with a female trainee. He struck a plea deal for 90 days of confinement. He later acknowledged being involved with a total of 10 trainees, a number previously unknown to investigators.

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News from © The Associated Press, 2012