The U.S. soldier charged with murdering 17 unarmed Afghan civilians last month is refusing to participate in an examination aimed at determining his mental state, according to his lawyers.

Staff Sgt. Robert Bales was due to be examined in what’s called a “sanity board” examination by Army doctors from Walter Reed Army Medical Center, seeking to evaluate his mental health and find out what his mental state might have been at the time of the massacre.

John Henry Browne, Bales’s attorney, said Friday he advised Bales to invoke his Fifth Amendment right to remain silent because the Army will not allow Bales to have an attorney present at the sanity board review.

“A member of the military does not give up constitutional rights by being in the military,” Browne wrote in an email to reporters. “Since the defense will have no way to know questions asked or answers given, Sgt. Bales’ civilian attorneys have instructed him to invoke his Fifth Amendment right to remain silent and NOT participate in the sanity board process, particularly since his Sixth Amendment right to counsel has been denied during the board process.”

Maj. Chris Ophardt said these examinations are typically carried out without the presence of lawyers. “They want to make sure the board can ask the questions they need to ask to make a fair determination, without any outside influence,” he said.

While Bales has been the only one charged with committing the massacre, eye-witnesses and surviving victims on the crime have claimed from the beginning that Bales did not act alone, but with several of his fellow soldiers. If that’s the case, its more unlikely that Bales acted due to any form of insanity, and this could possibly be a reason h has chosen to refuse examination.