Trayvon Martin Rally (by Vic Damoses) (Photo credit: dombrassey)

By Associated Press

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.

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.

Read More A question of civil rights? Federal role shaped by decades of cases before Trayvon Martin – The Washington Post.