The video opens with blood already soaking through the driver's shirt, and the police officer who shot him cursing, his gun still pointed at the dying man.
The driver's girlfriend - who watched the encounter and streamed the gruesome aftermath in live video to Facebook from the passenger seat - asks at one point for help.
Her plea brought scores to the street within hours in an angry protest that rolled through the day at the governor's mansion in St Paul, Minnesota, and across the country, echoed in a congressional hearing with the FBI director and brought a grim-faced President Barack Obama to the podium in Poland to call for greater urgency in police reform.
In Dallas, a downtown protest march in the evening turned violent, when two snipers shot at police officers, leaving five of them dead, the city's police chief said. Last night three people were in custody and a fourth person was exchanging gunfire with officers.
"This is not just a black issue. This is not just a Hispanic issue. This is an American issue," Obama said. The fatal police shootings are "symptomatic of a broader set of racial disparities that exist in our criminal justice system".
In the wake of this latest in a long string of police shootings - and less than 48 hours after another black man was killed by police in Baton Rouge, Louisiana, authorities in Minnesota voiced shock and sympathy and vowed justice.
"Nobody should be shot and killed in Minnesota for a tail light being out of function," said Governor Mark Dayton. "Would this have happened if those passengers would have been white? I don't think it would have."
Earlier, in front of protesters at the governor's mansion, the Democrat had tried to console relatives of the driver, Philando Castile, 32, who died on Thursday at a Minneapolis hospital. "I can't tell you how sorry I am that this terrible tragedy was forced upon your family," he said.
"I don't want you guys to say you're sorry!" shot back Castile's girlfriend, Diamond Reynolds, her retort echoed by the surrounding crowd. "I want justice."
Their terse and public exchange, captured on national television, encapsulated the frustrations of African Americans across the nation. Demonstrators rallied against police brutality, in front of the White House and in New York's Times Square, where police made several arrests. Protesters marched in Chicago, Philadelphia, Atlanta and Los Angeles.
In Dallas, gunfire broke out while hundreds of people were gathered to protest against the fatal police shootings this week. Local TV stations showed protesters marching along a street about half a mile from City Hall, when the shots erupted and the crowd scattered, seeking cover.
Civil rights activists have noted how police officers are rarely charged in fatal shootings and how, in many cases, key details often remain unknown, including the identities of many officers involved.
In the latest death, Reynolds and Castile were on the way home from getting him a haircut for his upcoming birthday, she told reporters, when they were stopped by police for having a broken tail light. Her 4-year-old daughter was in the back seat. It was dusk.
In the video stream she posted live, Reynolds said her boyfriend had just told the officer he had a legal firearm and was retrieving his gun permit and driver's licence from his wallet when the officer opened fire. Blood had already spread across Castile's white shirt, and he appeared to lose consciousness while the St Anthony police officer who shot him is seen in the background shouting: "I told him not to reach for it! I told him to get his hands up."
In response, Reynolds said: "You told him to get his ID, sir, his driver's licence." At one point, as Reynolds screamed in grief and frustration, her daughter could be heard trying to comfort her mother in a small voice, saying, "It's okay, Mommy. It's okay. I'm right here with you."
"Please don't tell me my boyfriend's gone," Reynolds pleads in the video. "He don't deserve this, please. He works for St Paul Public Schools. He's never been in jail, anything. He's not a gang member, anything."
Yesterday, Reynolds said the officer fired five times, authorities did not check Castile for a pulse and it took 15 minutes for paramedics to arrive.
"They killed him while he was still wearing his seat belt," she said. "The officer was jittery from the second he pulled us over until he pulled the trigger."
In his news conference, Dayton noted the lack of medical attention. "No one attended to his condition as they attended to the police officer involved. The stark treatment, I find absolutely appalling at all levels."
I don't want you guys to say you're sorry! I want justice.
In an interview at their family's home, Castile's sister, Allysza Castile, showed a reporter a black 9mm handgun, with a loaded magazine, that she was keeping perched near their front door.
"I'm scared of the police," she said. "They're slaying us like animals."
She said she had not slept since the shooting, and as she talked about her brother, she broke down in tears.
Allysza and Castile's mother, Valerie, said they raced to the scene when friends watching his girlfriend's Facebook stream started calling.
"The man was executed in that car," Valerie Castile said. "People need to be held accountable for what they do."
She believes the police officer profiled her son because he was black and wore dreadlocks. She said she had taught her son, described by the principal at the school where he worked as devoted to children, to always comply with police orders.
She said Castile owned a firearm to protect his home.
Minnesota state officials named Jeronimo Yanez as the officer who shot and killed Castile during the traffic stop. The state agency investigating the shooting said that Yanez and Officer Joseph Kauser, who have both been with the St Anthony Police Department for four years, were on administrative leave. The Minnesota Department of Public Safety said it has collected several videos of the shooting, though none from body-worn cameras on either officer.