Lyoya, a 26-year-old refugee from the Democratic Republic of the Congo, was killed by police at the end of a traffic stop in Michigan

Lawyers for the family of Patrick Lyoya, a Black man killed by police in western Michigan, have released results from an independent autopsy.

Video from a bystander showed that Lyoya, who was not armed, was on the ground when he was shot in the head during a struggle with a white Grand Rapids officer on 4 April.

The official autopsy report is being shared with state police and will not immediately be released to the public.

A separate autopsy was performed by Dr Werner Spitz, a 95-year-old forensic pathologist and former medical examiner who worked on investigations after the assassinations of John F Kennedy and Martin Luther King, among other high-profile cases.

In a press conference on Tuesday, Spitz held up a model of a human skull, in which he made a hole in the back of the head and inserted a probe through it.

“The bullet went through the bone and lodged under the skin … for the simple reason that the skin is elastic, which lost the velocity of the bullet,” Spitz said.

He added: “There was no complete exit, only an exit from the skull, not from the head. There is no question what killed this young man … the bullet entered in the back of the head.”

Spitz demonstrated the path of the bullet on himself, pointing to the back of his head and tracing the bullet wound to his right temple.

“It slid along the interior surface of the bone. The skull is sort of round in this location, so when the bullet travelled from the back of head to this location along the inner surface of the bone, until it hit this area of the bone, it broke it and lodged … partially inside the head and partially under the skin.”

At a press conference last week, the Grand Rapids police chief, Eric Winstrom, acknowledged that “Lyoya was shot in the head” but did not comment any further, saying: “That’s the only information I have.”

On Tuesday the civil rights attorney Ben Crump, representing Lyoya’s family, announced: “Today, based on scientific evidence, we can confirm that Patrick Lyoya was shot in the back of the head … [in] what his family believes was an execution.”

Lyoya’s death has outraged many who have watched video of the confrontation with an officer.

The officer, whose name has not been released, is heard saying during the traffic stop that the licence plate does not match the car. Lyoya declines to get back into the vehicle as ordered and a short foot chase ensues.

The officer ended up on top of Lyoya, trying to subdue him. He fired his gun after demanding that Lyoya drop his police Taser.

During a press conference last week, Lyoya’s father, Peter Lyoya, said: “At the time when I saw this video, my heart was really, deeply broken. Right now, I’m seeing that I have no life, my life has come to an end. My life was Patrick, my son.

“I’m asking for justice,” he added.

Crump said: “We demand that the officer who killed Patrick not only be terminated for his use of excessive and fatal force, but be arrested and prosecuted for the violent killing of Patrick Lyoya.”

Winstrom has said the name of the officer who shot and killed Lyoya will be released only if the officer is charged.

State police will give their findings to the Kent county prosecutor for consideration of any charges.

Patrick Lyoya was a refugee from DR Congo who came to the US in 2014 with his parents and siblings to escape violence. His funeral is planned for Friday at the Renaissance Church of God in Christ in Grand Rapids. The Rev Al Sharpton’s National Action Network said it would help to cover costs. Sharpton will deliver a eulogy.

Crump said that if charges are not issued against the officer, he and his co-attorney Ven Johnson would appeal to the state attorney general, Dana Nessel, and other authorities.