SAN ANTONIO — A prosecutor says a Texas Air Force base instructor on trial in a broad military sex abuse scandal posed as a "wolf in sheep's clothing."

Closing arguments were being held Friday in the court-martial of Staff Sgt. Luis Walker at Lackland Air Force Base in San Antonio. Walker is charged with 28 counts including rape and aggravated sexual assault involving at least 10 female trainees.

Lackland is where every American airman receives basic training. Prosecutor Maj. Patricia Gruen on Friday accused Walker of taking advantage of female trainees already terrified of their instructors. Gruen called Walker the "consummate predator."

Walker's defense team will present closing arguments later Friday.

Walker faces the most serious charges and prosecutors call his trial a cornerstone of the larger investigation.