A group of men chased a black jogger - with one warning "I'll blow your f****** head off!" - because they assumed he had committed a crime, a court has heard.
Ahmaud Arbery was pursued for five minutes by the group before being shot dead near Brunswick, Georgia, on 23 February 2020.
A phone video of the killing sparked outrage, and jurors were told on Friday that Mr Arbery had given the group no reason to suspect him of anything.
"They assumed that he must have committed some crime that day," prosecutor Linda Dunikoski told the court.
"He tried to run around their truck and get away from these strangers, total strangers, who had already told him that they would kill him. And then they killed him," she said.
The 25-year-old was chased by Greg McMichael, 65, and his son Travis McMichael, 35, who grabbed weapons and got in their truck as he ran though their neighbourhood.
The court was told that William "Roddie" Bryan, 52, a neighbour, joined in and recorded the video of Travis McMichael shooting Mr Arbery three times.
The chase began after another neighbour saw Mr Arbery wandering inside a home under construction, where security cameras had recorded him before, and called a police non-emergency number.
Greg McMichael told police that at one point he had shouted at Mr Arbery: "Stop or I'll blow your f****** head off!", the prosecutor said.
"All three of these defendants did everything they did based on assumptions - not on facts, not on evidence," Ms Dunikoski told the jury.
"And they made decisions in their driveways based on those assumptions that took a young man's life."
Mr Arbery's mother cried out and sobbed as the grainy video of the killing was played to the court.
The prosecutor described how it shows Travis McMichael raise his shotgun as Mr Arbery approaches and tries to run around the opposite side of the truck.
He is then seen stepping in front of the vehicle to confront the fleeing man.
Greg McMichael, a former investigator for the local district attorney, told police they suspected Mr Arbery was a burglar and were trying to make a citizen's arrest.
He said his son fired in self-defence after Mr Arbery attacked him with his fists and tried to take Travis McMichael's gun.
The men's lawyers say the neighbourhood was "on edge" over reports of thefts.
"It is a citizen's job to help the police, and the law authorises that," said Robert Rubin, a lawyer representing Travis McMichael.
Mr Rubin described Mr Arbery as "an intruder" who had been recorded four times "plundering around" a house under construction.
He called the footage of his death "a horrible, horrible video" but said his client had acted to protect himself after Mr Arbery refused to stop and lunged towards him and his gun.
"Travis McMichael is acting in self-defence," he told the jury.
"He did not want to encounter Ahmaud Arbery physically. He was only trying to stop him for the police."
Prosecutors insist Mr Arbery was just out jogging, had no weapons, keys or wallet on him - and had committed no crimes in the area.
Ms Dunikoski described him as an "avid runner" who often ran in the neighbourhood - less than two miles from his home.
"You're going to be able to see his Nike shoes," she told the court, "where he had basically no tread left on them whatsoever".
The lawyer said the owner of the half-built property - where Mr Arbery had been seen on previous occasions - believed he was using a water source to quench his thirst and that nothing was taken.
The case was largely ignored until the video was leaked online in May last year.
There has been controversy over the the jury, which is made up of 11 white people and one black person.
It took more than two weeks to select from more than 200 people - who were asked in detail what they knew about the case and how many times they had watched the video.
Prosecutors have objected to the final jury and said defence lawyers cut eight potential jurors because they were black.
The judge conceded there appeared to be "intentional discrimination".
However, he said state law limited his authority to intervene as the defence gave non-racial reasons for excluding the black candidates.
All three defendants have pleaded not guilty to murder, aggravated assault and false imprisonment.