BATH, Maine—In the months before Robert Card carried out Maine’s deadliest mass shooting, those around him feared he was sinking deeper into a dangerous mental health crisis.

He told his family he had been hearing voices, prompting them to contact police about the many guns he had access to. His Army Reserve commanders ordered him to spend time in a New York hospital after having a paranoid episode during a training trip there. A firearms dealer refused to sell Card a silencer after he disclosed his psychiatric troubles.