At least 16 people are believed to have been killed in the US town of Lewiston, Maine, and searches are underway for the shooter.
The shooting began at 18:56 local time (22:56 GMT) on Wednesday at a bowling alley and restaurant, police said.
Residents of Lewiston, as well as those in neighbouring Lisbon, were asked to shelter in place.
Police named Robert Card, 40, as a person of interest and said he should be considered “armed and dangerous”.
The authorities have not yet confirmed a death toll but CBS, the BBC’s US partner, has said the number is at least 16 after speaking to multiple law enforcement officials.
There have also been multiple but unconfirmed reports of at least 50 people injured.
Michael Sauschuck, commissioner in Maine’s Department of Public Safety, said there had been “multiple casualties”.
Lewiston has about 38,000 residents and is the second largest city in Maine after Portland.
Lewiston police said officers received reports of shots fired at two locations, a restaurant called Schengees and Sparetime Recreation, a bowling alley, apparently about four miles apart.
Riley Dumont told ABC News that her 11-year-old daughter was participating in a children’s bowling tournament at the bowling alley when she heard multiple gunshots.
She said her father, a retired police officer, then locked the family in a corner.
“I was laying on top of my daughter,” she said. “My mother was laying on top of me.”
She added that she saw three or four victims clearly.
Lewiston City Council candidate Billie Jayne Cooke told BBC News Channel she was leaving an event when details of the shooting came to light.
“Helicopters, sirens, I’ve never heard so much activity in my life in this city. We have police from all over the state, from out of the state, coming up,” she said.
“The whole city’s on lockdown. It’s horrible. You just don’t think that’s ever going to happen and it did.”
In a statement on X, formerly Twitter, Maine State Police said: “There is an active shooter in Lewiston”.
The Androscoggin County Sheriff’s Office released images of a person of interest, stating she is at large and asking for the public’s help in identifying her.
They show a bearded man wearing a brown sweater and carrying a gun entering a building.
Robert Card is a firearms instructor trained by the U.S. Army at a facility in the town of Saco, Maine, according to a state police news release viewed by CBS.
The news release also said he spent time at a mental health center in the summer of 2023 and threatened to carry out a shooting at the base.
Authorities also shared a photo of a white vehicle, said its front bumper was believed to be painted black, and asked anyone who recognized it to contact police.
Mr Sauschuck told reporters that Lisbon residents had been told to shelter in place because that was where the vehicle was located.
Central Maine Medical Center said it is responding to a “mass casualty shooting” and is coordinating with other area hospitals to treat the injured.
Lewiston Public Schools Superintendent Jake Langlais said in a statement that the district’s schools will be closed Thursday.
A statement from the Department of Homeland Security (DHS) said Secretary Alejandro Mayorkas has been briefed and is continuing to monitor the situation.
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 neighbours”.