According to the New York Post, the discovery was made at Card’s house in Bowdoin on Thursday.
Investigators entered the home as they executed a search warrant, and found the note – the contents of which have not yet been made public.
It comes as the Coast Guard began its own surveillance operation with a plane and boats out on Boothbay joining the Card manhunt.
The authorities also noted that Card’s registered car was found near a boat launch in Lisbon, a downtown area in Lewiston around eight miles away from where the shootings took place.
The Messenger reported that at a 15-foot boat believed to belong to Card was missing from the dock.
Card’s next moves have been dissected by insiders, including retired New York Police Department chief Robert Louden, who said the criminal could have fled into the woods.
Speaking to NBC, Louden continued: “From that parking area where he dumped his car there are four distinct possibilities.
“He killed himself. He got on a boat and went down that little river that’s there.”
The former cop added: “He went into another car or motorcycle or something that was waiting for him.
“Or there’s a hiking trail. He could be in the woods someplace. And so it’s like a spider’s web going out — and you’ve got to follow all those pieces of the web.”
Reports say Card enlisted in the military 21 years ago.
He was hospitalized at a mental health facility earlier this year, with medics noting bizarre behavior he had been exhibiting while at Camp Smith near the military academy at West Point.
Among his victims were members of Lewiston’s deaf community, taking part in a cornhole competition at Schemengees Bar & Grille – a location for Card’s grizzly crimes.
To mark the traumatic shootings, the White House called for the country’s flags to be flown at half-mast.
