Massachusetts National Guard Commander Joseph Carter has agreed to retire after military investigators found probable cause that he indecently assaulted a subordinate while the two were on a training exercise in Florida in 1984.

Carter, who has been on administrative leave since March, could not be charged criminally because the statute of limitations has long since expired on the 28-year-old incident.

Army investigators did not find probable cause that Carter raped the woman—a 23-year-old secretary at the time of the incident—as she alleged. But they found he probably touched her inappropriately, engaged in conduct unbecoming of an officer, and later made false claims about the incident. If Carter had not agreed to retire, Gov. Deval Patrick would have had to convene a court-martial to remove him.

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Carter, who plans to issue a statement, said neither he nor his lawyer has seen the Army report. He questioned how investigators could discount the rape allegation and nonetheless find probable cause that he assaulted the woman. “It’s incomprehensible,” said Carter.

Major General L. Scott Rice., who was chosen to run the agency while Carter was on leave, will continue to serve as the Guard’s interim Adjutant General until Carter’s permanent successor is named.

In a statement, Governor Deval Patrick said the report “raises serious concerns about the General’s actions and his response to the allegations. It is clear to me that General Carter can no longer serve as Adjutant General.”