A human rights group has reported that a suspect in a high-profile Air Force sexual assault case was found dead at a military detention facility.

According to the Center for Military Human Rights Korea on Monday, a senior master sergeant, who was indicted for inflicting secondary harm and threatening retaliation, was found unconscious at 2:55 p.m. on Sunday.

He was later transferred to a civilian hospital, where he was pronounced dead.

Denouncing the death as negligence from which the Defense Minister won't be free from responsibility, the group said the suspect's passing will impede efforts to uncover the truth in the now high-profile case.  

His trial was scheduled to begin on August 6.

Defense Ministry spokesperson Boo Seung-chan said an investigation into his death has already been launched.

The suspect was the superior of the master sergeant who reported a sexual assault that occurred after a dinner gathering in March arranged by the suspect. The woman was found dead by apparent suicide at her on-base residence on May 22.