WASHINGTON (AP) — 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.

A look at some cases through history:

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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:

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—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.

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HATE CRIMES

Over the years, Congress expanded what became known as "hate crime" law. Many states also adopted their own laws for crimes motivated by bias. And many cases with civil rights overtones were prosecuted under conventional murder and assault statutes.

Two of the most notorious state cases — the murders of Matthew Shepard and James Byrd Jr. — inspired further expansion of federal law, although it took more than a decade.

Shepard, a gay college student, was abducted and brutally beaten by two men who left him tied to a fence post in a remote field in Laramie, Wyo., in October 1998. Three white men chained Byrd, a black man from East Texas, by his ankles to the back of a pickup and dragged him to death on a country road in June 1998. Both cases ended with convictions on state murder charges.

Outrage over those attacks helped propel Congress and President Barack Obama to strengthen federal hate crime law in 2009 by increasing penalties and removing the requirement that the victim in a federal case be engaged in a specific federally protected activity. The law, named after Shepard and Byrd, also added crimes committed because of the victim's gender, disability or sexual orientation.

Some federal hate crime cases include:

— A Hasidic driver accidentally hit and killed a 7-year-old black boy in the Crown Heights section of Brooklyn in August 1991, sparking rioting. A black man, Charles Price, egged on a crowd of onlookers to "get the Jews." The angry mob set upon another Hasidic Jewish man, Yankel Rosenbaum. He was stabbed by black teenager Lemrick Nelson. Nelson was acquitted in state court of second-degree murder charges. The federal government followed with civil rights charges against Nelson and Price. After their first federal convictions were overturned on appeal, Price pleaded guilty and Nelson was convicted and sentenced to 10 years in prison.

— Six Shenandoah Valley, Pa., high school football players headed home from a block party encountered Luis Ramirez, 25, and his girlfriend in July 2008. A fight ensued. Federal officials said the teens yelled racial epithets and "Go back to Mexico" as they beat Ramirez. He died of head injuries. Brandon Piekarsky and Derrick Donchak were acquitted of most charges in state court. The federal government stepped in, and Piekarsky and Donchak were convicted under a federal law prohibiting housing discrimination, because they were trying to force Latinos out of Shenandoah. Donchak also was convicted of conspiring with local police to cover up the crime. Both were sentenced to nine years in prison. The city's police chief was sentenced to 13 months in prison for the cover-up.

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COLOR OF LAW

Many federal civil rights cases involve police or other authorities abusing their power under "the color of law." Unlike in hate crime cases, prosecutors don't have to prove that these civil rights violations were motivated by racism or other bias.

Still, the best-known convictions came in racially charged cases:

—Rodney King led law officers on a high-speed chase in March 1991 and, once stopped, was slow to obey their commands. Police reacted by kicking King, clubbing him with their batons and shocking him with stun guns, causing 11 skull fractures. A witness' video of white policemen pummeling a black man as he lay on the ground played over and over on national television. Four Los Angeles officers were charged with assault; a jury with no black members acquitted them. The verdicts sparked rioting that set Los Angeles aflame and cost 55 lives, prompting King's famous plea "Can we all get along?" The Justice Department charged the officers with civil rights violations. After a second trial, two were convicted and sentenced to 30 months in prison. Two were acquitted.

— In the chaotic aftermath of Hurricane Katrina in New Orleans, police gunned down 17-year-old James Brissette and 40-year-old Ronald Madison, who were unarmed, and wounded four others as they tried to cross the Danzinger Bridge to what they hoped was safety. Police officers planted a gun, fabricated witnesses and falsified reports to cover up the Sept. 4, 2005, shootings. A Louisiana district judge threw out murder and attempted murder charges against seven officers after ruling that secret grand jury testimony had been wrongly used against them. The Justice Department moved in with a civil rights investigation and won prison terms ranging from 38 to 65 years for four officers involved in the shooting; other officers were sentenced in the cover-up.

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NO FEDERAL CHARGES

Prosecuting civil rights cases isn't easy. Just because the U.S. Justice Department investigates the possibility, such as in the Trayvon Martin shooting, doesn't mean a case will move forward. Prosecutors may not believe there is enough evidence that an attack was motivated by bias or that police officers willfully violated someone's rights. These investigations can take months, even years.

Some cases the Justice Department investigated under great public pressure but hasn't prosecuted:

— The March 1991 slaying of a black teenager in Los Angeles bears striking similarities to the Martin case. Korean-American grocer Soon Ja Du suspected that 15-year-old Latasha Harlins intended to steal a bottle of orange juice. The two got into a physical altercation, and Du fatally shot the girl. Police said the money for the juice was in Latasha's hand when she died. Du claimed self-defense. Unlike the Trayvon case, the incident was recorded by security cameras, which showed Latasha turning away seconds before she was shot. Du was convicted of voluntary manslaughter and sentenced to probation and community service. Anger over the light sentence touched off protests and fed racial tensions that boiled over in the 1992 LA riots. Under pressure to bring a civil rights case, the Justice Department opened an investigation, but Du wasn't charged.

— African immigrant Amadou Diallo, 22, was unarmed when he was gunned down outside his Bronx, N.Y., apartment in February 1999 by four white plainclothes police. They were part of an elite street crime unit and said they approached Diallo because he resembled a rape suspect they were seeking. As Diallo reached for his wallet, the officers fired a fusillade of 41 bullets. They thought he was reaching for a gun, the police said. The four officers were cleared in state court. The Justice Department decided there wasn't enough evidence for civil rights charges, which would have required proving the officers intentionally used excessive force.

—Sean Bell, a 23-year-old black New Yorker, was killed at the wheel of his car early on the morning of what would have been his wedding day. Police fired 50 shots as Bell and two friends were driving away from his bachelor party at a Queens strip club in November 2006. The officers had seen Bell's friends arguing with another patron outside the club and said they thought Bell's group planned a drive-by shooting. There was no gun in the car. Three officers — one black, one white and one Hispanic — were tried before a judge, who cleared them. The Justice Department found insufficient evidence to bring civil rights charges.

—On New Year's Day 2009, subway passenger Oscar Grant was killed by a transit police officer in Oakland, Calif. Cellphone cameras captured the scene — an unarmed, 22-year-old black man shot while lying face down on a station platform, surrounded by police. Outrage over the incident led to riots in Oakland. The shooting is recreated in the current movie "Fruitvale Station." Transit police had seized Grant and others while investigating reports of a fight on a train. Then-officer Johannes Mehserle said he meant to reach for his Taser and mistakenly pulled his gun. Mehserle, who is white, was convicted of involuntary manslaughter and sentenced to two years. The Justice Department announced in 2010 that it would look into a possible civil rights case, and the department says that investigation is still ongoing.

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Associated Press researcher Monika Mathur contributed to this report.

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