People could be seen running from the scene of one of the shootings in Lewiston
Speaking to reporters, Michael Sauschuck, commissioner in Maine’s Department of Public Safety, said the shooting had begun at 18:56 local time (22:56 GMT) and that there had been “multiple casualties”.
The Central Maine Medical Center said it was reacting to a “mass casualty, mass shooter event” and was coordinating with other hospitals in the area to treat the injured.
The Androscoggin County Sheriff’s Office released two images of a suspect, saying he was at large and asking for the public’s help identifying him.
They showed a bearded man in a brown sweater carrying a firearm walking into a building.
The man identified as a person of interest is a firearms instructor who was trained by the US military at a facility in the city of Saco, Maine, according to a state police bulletin seen by the Associated Press.
The bulletin also said he had spent time in a mental health centre in the summer of 2023 and had threatened to commit a shooting at the base.
Police also shared a photo of a white vehicle, saying its front bumper was believed to be painted black, and asked anybody who recognised it to contact police.
Mr Sauschuck said residents in Lisbon had been asked to shelter in place because that was where the vehicle was located.
Lewiston Public Schools Superintendent Jake Langlais said in a statement that schools in the district would be closed on Thursday.
A statement from the Department of Homeland Security (DHS) said Secretary Alejandro Mayorkas had been briefed and was continuing to monitor the situation.
“DHS is working closely with our federal, state and local partners to support the Lewiston community,” it said.
A US justice department statement said that federal agencies were assisting state and local law enforcement.
The White House said President Biden had spoken individually by phone to Maine Governor Janet Mills, Senators Angus King and Susan Collins, and Congressman Jared Golden.
Senator King said in a statement he was “deeply sad for the city of Lewiston and all those worried about their family, friends and neighbors”.
Lewiston has a population of around 38,000 people and is the second-largest city in Maine after Portland.