Documents released on Monday show it was one of a number of attempts to alert authorities to the danger Robert Card potentially posed.

In July, the firearms instructor, who was also a petroleum supply specialist and Sergeant First Class in the US Army reserve, spent two weeks in a psychiatric hospital before being released.

Around three months ago, Card tried and failed to buy a silencer from a gun shop owner who feared he might pose a danger to others.

And a deputy from the Sagadahoc County Sheriff's Office went to Card's home after a soldier from his reserve unit told them he feared Card was "going to snap and commit a mass shooting" because he was hearing voices again.

The officer made numerous attempts to find Card but could not locate him.

The sheriff's office was also told he was a firearms instructor and had access to weapons.

Papers published by the sheriff revealed that Card, 40, also avoided detection by police, the FBI, and the government, despite numerous warnings about his mental health and weapons skills raised over a period of around seven months.

Eventually, a state-wide alert was issued to warn authorities that he was known to be "armed and dangerous".

His army reserve unit then restricted weapons access and declared him "non-deployable", barring him from checking out firearms and other weaponry.

But despite these measures, 38 days after his weapons ban, Card killed 18 people and injured 13 after opening fire in a busy bar and bowling alley in the town of Lewiston.

His attack last Wednesday was the 36th US mass killing this year.

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Card's body was found on Friday after a tense, two-day manhunt.

In spite of all the complaints raised about him, he was found to have a stash of weapons in nearby Lisbon Falls, which were all bought legally, authorities believe.

Maine's politicians will look again at the state's gun laws, governor Janet Mills said on Monday.

Police are still refusing to confirm if Card's mass shooting was planned.