SAN ANTONIO -- The number of instructors at a Texas Air Force base under investigation in a widening sex scandal has increased from 12 to 15 and the number of alleged victims has also gone up by seven to 38, officials said Thursday.

Officials declined to identify the three new instructors at Lackland Air Force base in San Antonio under investigation or comment on the nature of the allegations against them because no charges have been filed against the three, said Brent Boller, a spokesman for Joint Base San Antonio, which operates Lackland.

The increase in the number of instructors and alleged victims was first reported by the San Antonio-Express News. The numbers could continue to grow.

"There may be others as we continue the investigation," Boller said.

Last week, a military jury sentenced one former instructor, Staff Sgt. Luis Walker, to 20 years in prison after he was convicted of rape and sexual assault. The counts against Walker were the most severe in the investigation.

Lackland is where all Air Force recruits go through basic training. It has about 500 instructors for about 35,000 airmen who graduate every year. While one in five recruits are female, most instructors are male.

Six of the 15 instructors being investigated have been charged, with Walker and another instructor, Staff Sgt. Peter Vega-Maldonado, already having been court-martialed. Vega-Maldonado admitted in June to having sex with a female trainee and he was given 90 days of confinement as part of a plea deal.

Courts-martial have been set for four trainers, with Tech. Sgt. Christopher Smith going to trial Tuesday at Lackland, Boller said.

Smith is accused of trying to develop an intimate relationship with a trainee, making sexual advances toward her, having illicit social encounters with a second woman and obstructing justice. He faces up to a year in jail and dismissal from the Air Force if convicted, Boller said.

Three other trainers will go to trial in September and October.

A two-star general, Maj. Gen. Margaret H. Woodward, has launched a separate, independent probe. Nearly 80 members of Congress have called for a hearing.

The House Armed Services Committee announced this week that it would hold a closed-door briefing next week on the ongoing investigation.

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.

The Air Force has permanently removed Walker and 35 other instructors for a variety of reasons that include misconduct, failure to meet standards and medical issues.

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