(CNN) -- Two more trainers at Lackland Air Force Base in Texas have been charged with sexual misconduct with cadets they trained, Air Force officials said Wednesday, in a growing scandal being investigated across multiple Air Force bases.
The new charges bring to six the number of instructors charged in the widening scandal. In addition, six other trainers there are also under investigation.
"There's a cabal of persons at Lackland that are sexual predators," said U.S. Rep. Jackie Speier, D-California, who has been following the issue closely. "They are people that feel that it's OK to train recruits and then force them to have sex with them," she said.
The commander of Air Force training has named two-star Maj. Gen. Margaret Woodward to lead a two-month inquiry, covering all four Air Force bases that handle basic training, which are located in Texas and Mississippi.
At Lackland, near San Antonio, one trainer has pleaded guilty to having an improper relationship with a trainee, as part of a plea agreement.
Staff Sgt. Peter Vega-Maldonado has been sentenced to 90 days in prison, 30 days hard labor and a demotion in rank and pay. In his plea deal, he also provided testimony against two other trainers who have been charged. He also said he had inappropriate relationships with 10 trainees.
The most serious charges against an instructor are the 28 counts, including rape and sexual assault, alleged against Staff Sgt. Luis Walker. His trial is set to begin July 16, and he has pleaded not guilty.
"Sgt. Walker is innocent until proven guilty in a court of law," said attorney Joseph Esparza, who pointed out that his client must have had a good record to have been selected as a trainer. "The charges alone are not proof of anything."
After Walker was accused last summer, Air Force officials say, other trainers came forward with additional allegations.
"They are doing a good job self-policing," said Col. Polly Kenny, a legal spokeswoman for the command. "The 97% or more of the instructors who are good folks have come forward and reported on other instructors they feel are being unprofessional and violating the law."
"We will aggressively investigate, to turn over every stone we can," she said. "Even though obviously those aggressive investigations are bringing up more and more cases, it's the right thing to do."
All six cases involve trainers and cadets, although in some cases the cadets were no longer in the training program.
"The basic training environment in particular is honestly a target-rich environment for sexual predators," says Anu Bhagwati, a former Marine now with the Service Women's Action Network. She said a young recruit undergoing boot camp could be particularly vulnerable and have difficulty seeking help if misconduct occurs.
"You're broken down and then built back up. The interaction you have with your instructors is so intimate and so frightening," she said. "You have that relationship which is based on fear and intimidation. If that's the person you're asking help from, it becomes a very bizarre scenario."
During the Lackland proceedings, one cadet said of the military training instructors, "You don't say no to them."
The Lackland situation is not the first time the military has faced a scandal involving trainers and their trainees. In 1996, a dozen officers at Aberdeen Proving Ground in Maryland were charged with sexual assault on female trainees.
Last year, the Defense Department recorded just over 3,000 cases of sexual assault throughout all its branches, but it estimates there may have been 19,000 or more. If true, that would mean the vast majority of incidents stay secret, and only 8% of them result in a court-martial.
"For everything you hear, there are usually many stories that you don't," said Bhagwati. "There's usually not just one perpetrator or two perpetrators. There are usually several perpetrators -- including the commanders who have allowed those perpetrators to co-exist in those units."
Speier says commanding officers should not be handling cases of sexual misconduct involving their own subordinates.
"We have to take it out of the chain of command, and we have to give a separate office within the military the authority to investigate and prosecute these cases."
Defense Secretary Leon Panetta in April announced plans to create a special victims unit to investigate and prosecute allegations of sexual assault.
CNN's Chris Lawrence contributed to this report