"The weapon believed to have been used in the attack was a sniper rifle with .308 caliber bullets, and it was purchased legally this year, officials said."
"Maine court records show that a man named Robert Card who was born on the same date as the person of interest was charged with speeding in 2001 and 2002. No other criminal records were listed in the state's electronic court records system or in several other public records databases."
But also:
"It added that law enforcement said Card 'recently reported mental health issues to include hearing voices and threats to shoot up the National Guard Base in Saco, ME.'
The bulletin said Card was reported to have been committed to a mental health facility for two weeks this summer and then released. NBC News has not been able to independently verify the bulletin's statements about Card's history."
In previous incidents, people committed to mental health facilities didn't have it turn up on their background check unless it was ordered by a judge. That needs to change.
I'm seeing varying reports that he was also convicted of domestic abuse, but this link shows no such charges.