Horrified neighbours initially thought the headless body of a woman found in Long Island, US was part of a Halloween prank. Courtesy Fox News

A NEIGHBOUR who knew the psychologically disturbed man who beheaded his mother before throwing himself in front of a train has provided an insight into their unusual relationship.

Derek Ward, 35, allegedly hacked off the head of his mother, Patricia Ward, 66, with a knife and kicked it across the street before throwing himself in front of a moving train in Long Island, New York, on Tuesday night.

As the city struggles to make sense of the grisly incident, it has been revealed that Mr Ward had a long history of mental illness and a number of run-ins with the law, according to the New York Post.

Mr Ward, who was unemployed, lived with his mother, a respected languages professor, in a Long Island apartment.

A former neighbour has described their relationship as unusual.

“I knew something wasn’t exactly normal, but I don’t like to judge … You’d notice they were always together. I didn’t know if he lived with his mom because he was in some sort of mental state,” the neighbour told the Post.

“I did notice that it did seem she was very protective, making sure he had certain things. I’d hear her ask, ‘Do you have your wallet?’ — that type of thing.”

Detective Lieutenant John Azata said Mr Ward has a psychiatric history that stretched back 10 years.

“It exacerbated itself within the last year … It just seemed to be he was acting out,” he told the Post.

Police said there were no witnesses to the murder, and that they are trying to determine a motive. His mental illness could have been intensified by the stress of his paternal grandfather’s recent death, according to CBS New York.

Mr Ward was arrested for criminal mischief in 2003, but was released on probation. In 2006, he was jailed for 45 days for illegal possession of a gun and 100 Valium pills.

Farmingdale,NY-Woman reportedly decapitated, male found dead under train after possible murder/suicide #breakingnews pic.twitter.com/sXwQnbCMFW — Stringer News (@Stringernews) October 29, 2014

He is believed to have decapitated his mother inside their Secatogue Ave apartment Tuesday night before dragging her remains out onto the street. She was found with broken ribs and multiple stab wounds.

Witnesses said he kicked the head about 6m down the street. Because the remains lay near a home decorated with pumpkins, fake cobwebs and a mock graveyard, they initially thought was a Halloween prank.

Man jumps in front of LIRR train after allegedly beheading woman in Farmingdale http://t.co/6vLb4PfOqO via @pix11news pic.twitter.com/WliOe3uuRr — Lisa Daftari (@LisaDaftari) October 29, 2014

New York man beheads woman, then commits suicide by train http://t.co/BFmoL0PXmt via @NBCNewYork pic.twitter.com/HURqDb4NAK — NBC News (@NBCNews) October 29, 2014

“I literally thought it was a prank,” one witness told WCBS 880. “We saw the body on one side and saw the head on the other side. I thought everything was a prank.”

Neighbours only found out the horror was real when they tried to lift the lifeless body, according to the Post.

“I looked though my window and saw the body down there,” neighbour Nick Gordon said.

“I saw the lady laying right in front and her head was across the street, close to the corner. I thought, ‘holy s***!’”

He said there was a blood trail from the apartment block to the street outside.

“There was blood all over the floor. You can see smears going down the stairs, as if someone was pulling a body,” he said.

About 25 minutes after the murder, Mr Ward jumped in front of an eastbound train on the Long Island Rail Road line.

Professor Ward’s severed head and body were found on Secatogue Ave, Farmingdale, about 8pm; and Mr Ward’s dead body was found about 20 minutes later, 1.6km from the murder scene.

Professor Ward taught at Farmingdale State College for nearly three decades, and a spokesman for the institution described her as “well-known, well-liked and well-respected”.

“You know, a lot of tears were shed on campus today,” he said.

Police said the incident was related to domestic violence, not terrorism.