Briones's mother, Susie, told The Fresno Bee that her son hit his breaking point during the April incident.
His best friend was killed Nov. 19, the day of the attack in the western Iraqi city of Haditha, and he was still grieving when he was sent in to clean up the bodies of the Iraqi civilians.
Susie Briones said her son told her he saw the bodies of 23 dead Iraqis that day. Twenty-four were slain.
US authorities have launched two investigations, one into the deadly encounter itself and another into whether it was the subject of a cover up.
Ryan Briones is seeing a psychologist in San Diego to help deal with the stress, and military officials are aware of his deteriorating mental health, his mother said.
Calhoun said he was aware of Briones' situation, adding that the Marine has agreed to be evaluated by a psychiatrist for post-traumatic stress and whether it played a role in the alleged crime.
Briones, who has returned to Camp Pendleton, is scheduled to appear in court June 19.