WASHINGTON - Almost as soon as George Zimmerman was pronounced "not guilty" in a Florida courtroom, the cry went up.
The U.S. government must get "justice for Trayvon," insisted protesters angry about the fatal shooting of unarmed black teenager Trayvon Martin. The call will resound again later this month through events marking the 50th anniversary of the March on Washington.
Attorney General Eric Holder, the first black man to lead the nation's law enforcement, says the Justice Department is investigating.
In this file photo a pedestrian passes a mural of Oscar Grant in Oakland, Calif., shortly before a jury delivered an involuntary manslaughter verdict in the trial of a former transit police officer, Johannes Mehserle, who shot and killed subway passenger Grant as he lay unarmed and face down on a station platform on New Year's Day 2009.
Why would the feds consider stepping into a state murder case?
The federal government has claimed its power of protecting civil rights against violence as far back as the Reconstruction era. Empowered by constitutional amendments and early civil rights laws passed after the Civil War, the government sought to protect newly freed blacks and their voting rights, mostly from the Ku Klux Klan.
But then court decisions, the end of Reconstruction and the rise of Jim Crow laws essentially "defanged" the federal government of its power to police civil rights when state and local governments would not, said Darrell Miller, a Duke University law professor.
It wasn't until the 1960s civil rights movement - exemplified by the historic Aug. 28, 1963, march - that new laws began strengthening the federal role.
Now, the Justice Department is expected to pursue civil rights prosecutions. But in many cases that inflame racial passions, federal prosecutors don't find the evidence needed to support civil rights charges.
A look at some cases through history:
THE CIVIL RIGHTS ERA
As the burgeoning civil rights movement gathered force in the 1960s, demonstrators were brutalized and killed, sometimes at the hands of law officers. Many slayings remain unsolved. But in some cases where local authorities failed to go after the attackers or all-white juries refused to convict, the federal government moved in with civil rights charges.
The strategy won federal convictions in some racist killings that had jolted the nation:
-The 1964 slayings of three young civil rights workers - James Chaney, who was black, and Andrew Goodman and Michael Schwerner, who were white - that would later inspire the movie "Mississippi Burning."
-The shooting death of Lt. Col. Lemuel Penn, a black World War II veteran, by Ku Klux Klan members as he was driving home from Army Reserve training in Georgia in 1964.
- Fatal shots fired in 1965 into the car of Viola Gregg Liuzzo, a white activist who was helping shuttle black demonstrators between Selma and Montgomery, Ala.
These cases helped build public support for strengthening federal law enforcement's hand through a series of civil rights laws. Notably, the Civil Rights Act of 1968 made it a U.S. crime to use threats or violence to interfere with someone's employment, housing, travel or any of several other federally protected rights because of that person's race, religion, color or national origin.