image copyrightSuffolk County District Attorney's Officeimage captionNathan Allen was shot dead by police after the attack

An American man who crashed a stolen lorry into a house before shooting two black bystanders was a suspected white supremacist, police say.

Nathan Allen, 28, fatally shot retired policeman Dave Green and military veteran Ramona Cooper in Saturday afternoon's attack in Massachusetts.

Investigators later found racist and anti-Semitic writings by Allen, who was shot dead by officers at the scene.

He was married, employed and had a PhD and no criminal history, police said.

According to CBS affiliate WBZ-TV, Allen walked through a marsh to a garage in the city of Winthrop where he stole a plumber's lorry. He crashed the vehicle into an unoccupied home, causing extensive damage.

He then climbed out of the wreckage and attempted unsuccessfully to carjack another vehicle.

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As he walked away, he shot Ramona Cooper, 60, a staff sergeant in the Air Force, three times in the back, killing her.

He then shot retired Massachusetts state trooper Dave Green, 68, multiple times. When police arrived on the scene they exchanged gunfire with Allen, killing him.

Investigators said they found "troubling white supremacist rhetoric" in the gunman's handwriting.

Suffolk District Attorney Rachael Rollins told a press conference on Monday that Allen is believed to have acted alone.

The prosecutor said he had "likely appeared unassuming" before unleashing his attack with a firearm that he legally owned.

image copyrightSuffolk County District Attorney's Officeimage captionRetired police officer Dave Green and military veteran Ramona Cooper were both killed

Her office's statement said: "This individual wrote about the superiority of the white race. About whites being 'apex predators'. He drew swastikas."

The statement added: "He stole a box truck, crashed it into another vehicle and a property, walked away from the wreckage interacting with multiple individuals and choosing only to shoot and kill the two black people he encountered."

In a tweet on Monday, Massachusetts Governor Charlie Baker called the killings "a despicable act".

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