The US justice department is to launch a civil rights investigation into the death of Eric Garner, a black man who was placed in an apparent chokehold by a white New York police officer.
The inquiry was announced by Attorney General Eric Holder after a grand jury decided against charging the officer.
That decision prompted streets protests in New York. Activists have called for a march in Washington next week.
President Barack Obama said the case “speaks to larger issues”.
The decision not to charge the officer came just a week after another grand jury in Missouri did not charge a white police officer who killed a black teenager in Ferguson, sparking riots there and protests across the country.
This week, in light of the Ferguson protests, President Obama sought emergency funding to improve police training and to restore trust in policing.
‘Fair and expeditious’
The New York case sparked attention when a video of Officer Daniel Pantaleo arresting Mr Garner surfaced in July.
In the incident, asthmatic Garner, 43, was heard on the video shouting “I can’t breathe!” as a number of officers restrained him on a street in New York.
That cry was echoed by protesters who marched through the streets in New York late on Wednesday, occasionally disrupting traffic.
Attorney General Eric Holder announced “an independent, thorough, fair and expeditious” investigation into potential civil rights violations in the case.
He also said the Department of Justice would conduct a “complete review” of material gathered in the local investigation. “All lives must be valued – all lives,” he said.
“Mr Garner’s death is one of several recent incidents across our great country that have tested the sense of trust that must exist between law enforcement officers and the communities they are charged to serve and to protect.”
He urged those who planned to demonstrate against the grand jury decision to do so peacefully, and said he was continuing a review of how to heal a “breakdown in trust” between law enforcement officers and communities.
In isolation, the decision of the grand jury in Staten Island not to indict the white NYPD officer Daniel Pantaleo would have sparked anger.
The fact that it came less than 10 days after a grand jury in Missouri decided that the white officer involved in the shooting of Michael Brown should not face criminal charges has amplified the sense of racial injustice felt by those who believe the decision is inexplicable.
In contrast to Ferguson, there is video evidence showing what happened in Staten Island. New York’s medical examiner had already ruled that the death of Eric Garner was a homicide, and that the chokehold contributed to it.
Even though America has a black president and a black attorney general, Eric Holder, this will reinforce the widespread feeling in poor African-American communities that the criminal justice system is weighted against them, and that the law is not colour-blind.
After the grand jury decision President Obama said: “When anybody in this country is not being treated equally under the law, that is a problem, and it’s my job as president to help solve it.”
New York Mayor Bill de Blasio called the outcome “one that many in our city did not want” and appealed for a “peaceful, constructive” response.
‘Fight to the end’
Garner family lawyer Jonathon Moore said he was “astonished” by it, and Eric Garner’s widow, Esaw Garner, said her husband’s death would “not be in vain”.
“I’m determined to get justice for my husband, because he shouldn’t have been killed in that way,” she said. “As long as I have a breath in my body, I will fight the fight to the end.”
Mr Garner’s daughter, Erica Snipes, told the BBC she was outraged.
“On that video you can see the most cruel horrible thing that someone could do to someone,” she added. “It’s just not right.”
Civil rights activists Al Sharpton called for a march in Washington on 13 December. “We have no confidence in local state prosecutions because state prosecutors work hand in hand with the local police,” he said.
Mr Pantaleo issued a statement in which he said it was never his “intention to harm anyone” and that he was praying for Garner and his family.
Mobile phone video of the incident shot by a witness showed Garner verbally refusing to be handcuffed. Mr Pantaleo restrained him, holding him by the neck.
The city’s medical examiner’s office said Garner’s death was caused by “the compression of his chest and prone positioning during physical restraint by police”.
But it said other factors contributing to his death included asthma and heart disease.
Mr Pantaleo’s lawyer had argued he had used a move taught by the police department, not a chokehold which is banned under New York Police Department policy.
Following Garner’s death, New York Police Commissioner William Bratton ordered officers at the nation’s largest police department to undergo retraining on restraint.