The BBC's Clive Myrie: " One in six police shootings are prompted by people wanting to die"

The study shows that last year one in six of all police shootings were the result of people engineering their own deaths.

US Police forces say they had already recognised the phenomenon - known as "suicide by cop" - but this is the first study.

Clive Myrie reports from New York: "The effect on police officers can be devastating"

"Law-enforcement officers are chosen because they always carry a weapon, and if they find themselves in a situation where they have to use deadly force, they will use deadly force," said Dr Range Hudson of Harvard University, who worked on the study.

"A suicidal individual knows this and will escalate the level of violence to make that happen."

Suicidal hijack

Last year two members of a religious cult hijacked a bus on an Arkansas highway in order to stage-manage their suicides, the research found. They wanted to force armed officers to kill them.

The man and his wife left the bus and walked towards police lines with their guns drawn. They then suddenly stopped and kneeled down in the middle of the road to increase the likelihood they would be killed, not wounded.

In full view of television cameras, they raised their guns to shoot at the police, who were left with no other alternative. The hijackers died instantly.

Third leave a note

In a third of the cases, those committing "suicide by cop" have left behind notes explaining their actions.

Moshe Pergament, 19, provoked an officer in New York to shoot him in November last year.

In his suicide note, he made clear that it could have been anyone on the beat out of a force of thousands who pulled the trigger.

"Dear Officer, it was a plan... I'm sorry for getting you involved. Please remember that this was all my doing. You had no way of knowing," Mr Pergament wrote.

Police guilt

Nevertheless, the effect on a police officer of killing someone in such circumstances can be devastating. Some leave the force, others have to live with the guilt.

Sergeant Steve Kaneip faced a man wielding two handguns in a tense stand-off 18 months ago in North Carolina.

The man had repeatedly told negotiators that he wanted to die and that he had to make the police do their job.

"When he brought his guns to bear on everybody else who was standing in front him ... I had to fire," Sergeant Kaneip says.

"Every day when I look in the mirror I have to live with it. It turned out to me to be the worst nightmare," he says.

Training encouraged

In Los Angeles, officers are being trained to deal with suicidal suspects. Researchers say they hope their study will encourage more police departments around the country to do the same.

"If you are back a little further, if you have got better cover, then you may have a little more time to be able to evaluate that situation," says Dr Audrey Hoenig of the Los Angeles County Sheriff's Department.