Mass murderer Robert Card, who killed at least 18 people in a gruesome massacre in Lewiston, Maine, Wednesday, was reportedly on state authorities' radars as early as mid-September.

Card gunned down at least 18 people and wounded 13, after opening fire on a bowling alley and bar in Lewiston, causing the worst mass shooting in the Pine Tree State's history. He was found dead Friday night after a two-day search, with officials concluding that he shot himself in the head.

Law enforcement officials told the Associated Press that they were alerted to "veiled threats" by Card, a U.S. Army reservist, after he threatened soldiers at a southern Maine National Guard Base in Saco.

When authorities visited Card's home and couldn't find him, they dropped their investigation.

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"We added extra patrols, we did that for about two weeks," Saco Police Chief Jack Clements explained. "The guy never showed up."

"Never came in contact with this guy, never received any phone calls from the reserve center saying, ‘Hey, we got somebody who was causing a problem,’" he added. "We never got anything."

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Sagadahoc County Sheriff Joel Merry alerted every law enforcement agency in Maine after the Army Reserve informed his department, but also could not find Card after a welfare check to his residence.

"We couldn’t locate him," Merry said.

The FBI told AP that they were not aware of suspicious activity from Card, explaining they "did not have nor did [we] receive any tips or information concerning Robert Card."

"[The background check system] was not provided with or in possession of any information that would have prohibited Card from a lawful firearm purchase," the agency added.

The Associated Press contributed to this report.