A massive manhunt was under way Thursday for a gunman who a local official said killed at least 22 people and wounded "many" more in mass shootings in the US state of Maine, the deadliest such incident this year.

Police said Robert Card -- seen in surveillance footage pointing a semi-automatic weapon with an extended clip as he walked into a bowling alley in the town of Lewiston -- "should be considered armed and dangerous."

Card is a certified firearms instructor and a member of the US Army Reserve, CNN reported, citing law enforcement sources.

Lewiston city councilor Robert McCarthy told CNN that law enforcement had "confirmed 22 dead, many, many more injured", with local media reporting shootings had occurred at multiple locations.

Swathes of Lewiston were locked down, with businesses urged to shutter and people ordered to shelter in place, as the scourge of horrifying gun violence once again ripped through an American community.

Maine public safety official Mike Sauschuck said he was not prepared to give a death toll, calling it "a very fluid situation."

He told reporters police were flooding the streets as they sought the gunman. "We have literally hundreds of police officers working around the state of Maine to investigate this case, to locate Mr. Card," he told reporters.

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