A man confessed to killing four people, including his parents, and then firing on motorists on a busy interstate highway, just days after being released from prison, US police have said.

Law enforcement officials released more information, including identities of the victims of the shootings on Tuesday at a home in Bowdoin and 25 miles to the south on the highway in Yarmouth, Maine.

Joseph Eaton, 34, had been released on Friday from the Maine Correctional Centre in Windham, where he was picked up by his mother after completing a sentence for aggravated assault, police said.

That crime was serious enough to prevent him from possessing a gun in Maine.

A lone bouquet of flowers marks a makeshift memorial at the end of a driveway in Bowdoin, Maine (Rodrique Ngowi/AP)

The shootings in Maine began in the small town of Bowdoin, where four people were killed on Tuesday, with three bodies discovered in a home and one in a barn, police said.

Then a chaotic scene developed in which shots were fired at vehicles on Interstate 295 more than 20 miles away in the community of Yarmouth, police said.

Three people were shot there, and one remained in a critical condition on Wednesday.

Mike Sauschuck, commissioner of the Maine Department of Public Safety, called the crimes “an attack on the soul of our state” that shook neighbours, law enforcement officers and the state at large.

“It’s a shock to everybody,” he said.

“You want to naturally say, ‘That can’t be happening here in Maine’. But the reality is these senseless acts can and do happen everywhere.”

The victims in Bowdoin were identified as Eaton’s parents, Cynthia Eaton, 62, and David Eaton, 66, along with their friends, homeowners Robert Eger, 72, and Patricia Eger, 62, police said.

Police said Joseph Eaton told them he was firing on cars because he thought he was being followed by police.

Investigators work at the scene of a deadly shooting in Bowdoin, Maine (Robert F Bukaty/AP)

Several vehicles were hit by gunfire but the three people injured were a family all in the same car: Sean Halsey, 51, Justin Halsey, 29, and Paige Halsey, 25, police said.

Paige Halsey remained in a critical condition, police said.

The seven were the latest victims of mass shootings in the US, whose targets included a Christian elementary school in Nashville, Tennessee, a bank in Louisville, Kentucky, and a Sweet Sixteen party in a small city in Alabama.

“This is an active investigation with a lot of moving parts,” Shannon Moss, state police spokesperson, said on Wednesday.

The day before the shootings, an anguished man believed to be Eaton posted a roughly two-minute live video on Facebook criticising people who he said are Christian and do not give people a second chance.

“What good does it do to hate somebody?” he said, choking back tears on the video.

“You know, it destroys you.”

On the day he was released from prison, the man believed to be Eaton posted on Facebook that he was feeling thankful.

“It’s finally over. There are so many people I can’t wait to see.”

Maine Department of Public Safety Commissioner Mike Sauschuck leads a news conference in Augusta, Maine, about shootings in the state(Patrick Whittle/AP)

Ms Moss confirmed that state police were aware of the video, and that it is part of their investigation.

Eaton, who was living in Bowdoin, was charged with four counts of murder but was not immediately charged over the highway shootings, she said.

He was jailed while awaiting a court appearance.

It was unclear if he had a lawyer to speak on his behalf, a jail official said on Wednesday.

The names of the victims were not released, and state police did not discuss any possible motive.

The four bodies were taken to the state medical examiner’s office in Augusta for positive identification and post-mortem examinations.

Ian Halsey, of Bowdoinham, said that two cousins were shot and that his uncle suffered shrapnel injuries in a single car.

One of his cousins is in a critical condition, and none of the family knew the gunman, he said.

A police officer stands on the corner of Portland Street and Main Street in Yarmouth, Maine (Michael Leonard via AP)

“They were just passers-by in the wrong place at the wrong time,” he said of his family.

“It’s horrible what happened.”

In Bowdoin, Denise Pride, 58, a neighbour who lives in a farmhouse down the street from the location of the four deaths, said members of the rural community were shaken by the tragedy.

The house where the killings took place is in a wooded area of rolling hills and farms, and houses range from mobile homes to large estates.

Ms Pride said one of the victims was famous for delivering baked goods to neighbours on holidays.

“They were very kind people,” Ms Pride said.

“The neighbours were texting, shocked that it happened, and to them.”

Eaton was charged over the past decade with more than half a dozen crimes and served an eight-month sentence last year for assault, according to state records.

Past convictions included aggravated assault, a felony that would prevent him from legally having a firearm.

The origins and ownership of the firearms used in Tuesday’s shootings were unclear.

State police declined to comment on the weapon that was used.

Legislative Republicans issued the following statement in which they said they were “outraged by this senseless tragedy”.

“Incidents like this remind us of the depravity of some in our community who harm others. Yet, we’re also heartened by the response of our communities to pull together in trying times such as these, and the bravery and quick actions of our first responders,” the statement said.

In Bowdoin, yellow crime tape hung where the shootings took place in a home flanked by woods at the end of a long, gravel driveway.

Detectives and evidence technicians remained in the home collecting evidence late on Tuesday, long after hearses left the driveway.

In Yarmouth on Wednesday, traffic flowed normally on Interstate 295, where a day before the three people were shot in cars and the gunman was apprehended.