The suspect is believed to be Robert Card, a 20-year US Army veteran from Bowdoin Center, Maine.
(according to Telegrams so take with grain of salt)
EDIT: Name confirmed by news.
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According to law enforcement, CARD recently reported mental health issues to include hearing voices and threats to shoot up the National Guard Base in Saco, ME.
Preliminary reporting seems to suggest that it might actually be a mental illness thing. They reported to superiors that they were hearing voices and were discharged without any restriction on owning high caliber firearms.
Only in the army would nobody question the assumption that hearing voices automatically translates into always doing everything the voices tell you to do