WASHINGTON - The case files are piling up - young black men killed by white police officers who later go uncharged - as are questions about what the US government will do about it.
Names like Michael Brown, Tamir Rice and Eric Garner are now chapters in the long saga of tense and often violent relations between America's black community and mainly white police forces.
In a bid to reassure African Americans that something is being done, Attorney General Eric Holder, who is black, acknowledged Thursday that excessive force is routinely used by police in Cleveland, Ohio.
There, a 12 year old boy, Rice, was shot dead on November 22 by police responding to a report he was brandishing a gun in a park.
They opened fire seconds after arriving on the scene and killed him. Rice turned out to have been waving a toy pellet gun.
On Wednesday, after a grand jury said it would not indict a white New York cop for choking 43-year-old father of six Garner to death during an arrest, the top US law enforcement official announced a federal civil rights investigation of the case.
Holder promised a thorough, independent, timely and rigorous probe in an effort to calm the furious street protests that have erupted to demand justice for slain black Americans.
Law professor Paul Cassello cast doubt on how fast the wheels of justice might move.
"Sadly," he said, the Justice Department's track record in recent highly-publicized cases "does not inspire confidence."
Serving under America's first African American president, Barack Obama, the Justice Department has not issued any decision, for example, in its probe of the death two years ago of Trayvon Martin.
Martin was a young black man shot and killed in Miami by a neighborhood watchman who claimed he was acting in self defence after an altercation with Martin.
Holder has opened similar probes of possible civil rights violations in the death of Brown, who was shot dead by a white police officer in August in Ferguson, Missouri, and in the case of Rice in Cleveland.
In Ferguson, when a grand jury announced it had decided against indicting white officer Darren Wilson, and riots broke out, Holder quickly insisted that a federal investigation of the case and of the Ferguson police department in general was already underway.
But there has been silence over the Martin and Brown probes. Some kind of decision would foster confidence that the Garner probe and others will lead to some kind of result, Cassello said.
But "after that flurry of activity, for the next few months little was heard about the federal investigation," Cassello, who teaches at the University of Utah, told the Washington Post.
Reports in US media suggest that the Justice Department lacks evidence to file charges that the Ferguson killing and the Miami case were racially motivated.
"Federal civil rights law imposes an extremely high legal bar in these types of cases," Holder said.
Obama himself said he was determined to restore trust between the police and black communities.
The Us public has been shocked by video footage in recent cases of blacks who died at the hands of white police officers.
Garner was wrestled to the ground in a chokehold and Brown's bloodied body lay in the street for four hours in Ferguson. Security camera footage shows young Rice being shot dead in Cleveland.
'Storms will come'
"We are dealing with concerns that are truly national in scope and that threaten the entire nation," Holder said Monday at a Baptist church in Atlanta where Dr. Martin Luther King Jr. addressed black congregations more than 50 years ago.
"It was Dr King who reminded us - in his very last speech, on the night before his life was taken - that it's only when it is dark enough that the stars can be seen," Holder said.
"Bonds that have been broken must be restored. Bonds that never existed must now be created," said Holder, who has announced he will be leaving the Obama administration in a few weeks.
Holder, who wishes to leave an image as a champion of civil rights, has promised a reform of police behaviour, such as in Cleveland.
More than 20 federal investigations are under way and 15 agreements have been reached to correction police violations of people's constitutional rights.
"Our overall system of justice must be strengthened and made more fair," Holder said.
In nationwide initiatives the Obama administration is working to enhance ties between police and black communities and will soon announce new standards and "robust safeguards" to help end racial profiling -- the tendency of police to single out minorities for scrutiny.
"But the struggles will continue. The storms will come. And the road ahead will be anything but smooth or straight," Holder said.