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.
You don't like https://www.reddit.com/r/npd/ ?
The matter-of-fact tone the users there use always made me uncomfortable.
It doesn't feel productive.