Amateur video of San Francisco police surrounding a suspected knifeman and gunning him down in a hail of bullets has gone viral on social media and provoked outrage.
The short clip is the latest video posted online showing US police using what appears to be excessive force on suspects – many of them black and often with lethal effect.
The video, taken in broad daylight with a mobile phone by a passenger aboard a bus parked nearby, shows a man wearing a dark jacket walking and leaning against a wall as he is surrounded by at least seven police officers with their weapons drawn.
The suspect waves his hands, then puts them briefly in his pocket. A knife is not visible in the footage.
As the suspect slowly limps and continues to ignore commands from police, the officers open fire. The person taking the vision dives away from the bus window so the footage ends, but about 20 gunshots can be heard ringing out.
San Francisco police said that the dead suspect met the description given by witnesses of a person who had stabbed someone earlier that day. It was not clear when it happened.
An officer had tracked him down and quickly called for backup.
"The suspect was armed with a knife, believed to be the weapon used in the stabbing," police said in a statement.
"As the suspect had already demonstrated that he was a danger to others by having stabbed an earlier victim, the officers could not allow him room to harm anyone else."
Police said they ordered him to drop the knife "numerous times", but he failed to comply.
They fired non-lethal bean bags – used normally in crowd control – at him and doused him with pepper spray. The man fell to one knee, but did not drop the knife and tried to walk away.
An officer blocks him.
"Still armed with the knife, the suspect moved towards that officer," police said.
"At this point, fearing serious injury or death, officers fired their department-issued handguns at the suspect. The suspect was struck by the gunfire and fell to the ground."
Police in the United States have come under intense criticism for using excessive force against suspects.
Earlier this week Chicago's mayor fired the city police chief after a graphic video of a white officer fatally shooting a black teenager 16 times sparked a week of protests.