An air force instructor was sentenced to 20 years in prison on Saturday after being convicted of rape and sexual assault in a sweeping sex scandal that rocked one of the busiest military training centers in the US.

A military jury at Lackland Air Force Base in San Antonio found Staff Sgt Luis Walker guilty Friday night on all 28 charges he faced, including rape, aggravated sexual contact and multiple counts of aggravated sexual assault. A judge consolidated those charges Saturday into 20, but that didn't affect Walker's maximum sentence. He could have received life in prison.

Walker is among 12 Lackland instructors investigated for sexual misconduct toward at least 31 female trainees. Six instructors have been charged, on counts ranging from rape to adultery, and Walker was the first to stand trial. Walker also faced the most serious charges of all those accused.

Walker submitted a written statement during the sentencing hearing, but he also spoke from a podium directly to the jury while his two sons, ages seven and four, looked on quietly.

"I ask for my family's sake, for my two boys right there," Walker said, wheeling around and sobbing. "I ask that I am allowed to have a future with them."

Walker tried to mention having taken a polygraph test, but the prosecution objected before he could divulge the results.

"I promise you," he said of any sentence, "I'll accept it humbly and with the commitment to move forward positively."

Prosecutors say from October 2010 through January 2011, Walker sexually assaulted or had improper sexual or personal contact with at least 10 female recruits. Walker's court-martial included testimony from all 10 women, including one who described him luring her into an office and sexually assaulting her on a bed, ignoring her pleas to stop.

Five women testified during Saturday's sentencing hearing, saying they couldn't sleep or maintain relationships with men after the assaults. They said Walker's actions eroded their trust in authority and affected their performance at work.

One said it affected her tour in Afghanistan because she felt uncomfortable being alone with men.

"It's made it extremely hard to interact with authority figures," she said. "During my tour in Afghanistan, I was a little bit more scared of everything. I can't work with certain individuals just since they remind me of Staff Sgt Walker."

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

Walker was convicted by a jury of six men and one woman. Under air force rules, the jury can declare a defendant guilty by a three-fifths vote on each individual count, rather than the unanimous decision required in American civilian courts.