A black man has been shot dead by police in the US state of Minnesota as protests continued over the police killing of a black man in Louisiana.
Philando Castile's girlfriend live-streamed the St Paul incident's aftermath, showing him covered in blood as an officer pointed his gun at him.
He was shot as he reached for his driving licence, she said.
It follows the death of Alton Sterling, who was shot dead by police during an incident in Baton Rouge on Tuesday.
Hundreds of people have protested for two nights over Mr Sterling's killing.
The deaths follow a long line of high-profile incidents involving African-Americans dying at the hands of the police, igniting a national debate about the use of lethal force.
As news of the latest shooting spread, about 200 people protested outside the St Paul home of Minnesota Governor Mark Dayton, who later requested a federal investigation into the shooting.
In a statement on Facebook, President Barack Obama said all Americans "should be deeply troubled" by the two fatal shootings.
He said it was clear that they were not isolated incidents but "symptomatic of the broader challenges within our criminal justice system".
Black US tennis player Serena Williams, who is playing at the Wimbledon tournament, also expressed her dismay at news of Mr Castile's death, tweeting: "In London I have to wake up to this. He was black. Shot 4 times? When will something be done - no REALLY be done?!"
Mr Castile had been stopped in Falcon Heights, a suburb of St Paul, because the car had a broken rear light, the woman, identified in local media reports as Diamond Reynolds (or sometimes as Lavish Reynolds) said.
Before he was shot, he had told the officer that he was licensed to carry a concealed gun and had one in his possession, she said.
"You shot four bullets into him, sir. He was just getting his licence and registration, sir," Ms Reynolds says in the video.
A child, Ms Reynolds's daughter, was also in the car at the time.
Police said an investigation was under way and the officer involved had been put on leave.
Mr Castile's mother, Valerie Castile, told CNN that her son was just "black in the wrong place" and said there was "a silent war against African-American people".
Later on Thursday, an emotional Ms Reynolds told protesters outside the governor's house that she had filmed the incident so "the world knows that these police are not here to protect and serve us, they are here to assassinate us".
Mr Castile, 32, worked as a cafeteria supervisor at a Montessori school. His cousin Antonio Johnson told the Star Tribune newspaper he was "immediately criminally profiled" because he was black.
In Baton Rouge, hundreds of people gathered for a second night of protests at the shop where Alton Sterling, a 37-year-old father of five, was killed on Tuesday.
Police killings that scar the US
Walter Scott - unarmed and shot in the back as he ran away from an officer in North Charleston, South Carolina, in April 2015. Former officer Michael Slager facing murder charge
Laquan McDonald - 17-year-old was holding a knife but appeared to be moving away from police in Chicago when shot 16 times in 2014. Officer Jason Van Dyke denies murder charge
Michael Brown - 18-year-old shot at least seven times in Ferguson, Missouri, in August 2014, sparking nationwide protests. Officer Darren Wilson cleared of wrongdoing
Eric Garner - died after being placed in a chokehold by New York police while selling cigarettes in July 2014. Grand jury decides against charges, police disciplinary action taken against supervising officer Sgt Kizzy Adonis
A second piece of video from Baton Rouge emerged on Wednesday appearing to show Mr Sterling being held down and then shot several times, although some shots are heard when the camera moves away from the confrontation.
Seconds later, one of the officers is seen removing an object from the man's trousers as he lies on the ground with blood on his chest.
A witness said he saw officers take a gun from Mr Sterling's pocket after the shooting, but police have not commented on this.
Baton Rouge Police Chief Carl Dabadie Jr said Mr Sterling was armed but that a lot of questions still had to be answered.
The officers involved, Blane Salamoni and Howie Lake II, have been put on administrative leave and the US Department of Justice has launched a civil rights investigation.
Use of lethal force
- Officers can only justify firing their weapons at civilians if they fear the loss of life or limb
- Other options include verbal commands, use of empty hands to control a suspect, and use of less lethal weapons such as batons or pepper spray
- A 1982 Supreme Court case found it was illegal to shoot at fleeing felons