Relatives of an autistic 15-year-old boy shot dead by Illinois police in his own home are demanding justice.

Stephon Watts, who suffered from Aspergers syndrome, was shot twice in the head by police in his Calumet City home after his parents called them when he became aggressive.

Police claimed that Stephon, who they had regularly been called to placate, lunged at officers with a knife, giving them no choice but to use lethal force.

But Stephon's aunt, Petrell Powell-Bhoorasingh, who lives in Britain told IBTimes UK in an email that her brother, Stephon's father, was present throughout the whole event and claimed that the police were never under threat.

She said in her email: "[ His father ] stated that Stephon had his shirt over his head and had a butter knife which he did lash out with and my understanding of this is that with his shirt over his head he would not be able to see the police or anyone to harm them.

"How can they try to justify shooting him in his head twice for just having a table knife which is used on a daily basis for eating. They, by their own admission, knew his condition as have been dealing with him for the last 18 months."

She said she was shocked to hear that the officers had used a Taser to subdue Stephon on previous occasions.

"This has really taught us a very costly lesson - not to ever trust the police," she added.

Stephon's mother, Danelene Powell-Watts, was turned away from a press conference held at the Calumet City police station where she loudly berated the police over her son's death.

"[Danelene ] was unceremoniously turned away and told by the police to have a good day," said Powell-Bhoorasingh. "How patronising of them, knowing that they have killed her son in cold blood only a few hours ago.

"The picture that they are painting of my nephew and family is so wrong. They were called at 8.30am and by 9.48am he was pronounced dead at the hospital."

Calumet City police chief Edward Gilmore said officers did everything possible to avoid the use of lethal force.

"It's unfortunate that we had to get to this situation," he said. "We tried to do everything we could to keep him from being a victim, as he was an offender. He chose to be an offender."

He said that the officer at the scene felt his life was under threat and had "nothing else to do but defend himself".

The officers who entered the house have been placed on administrative leave.