The U.S. Justice Department could bring a hate crime charge against the man who shot and killed Florida teenager Trayvon Martin.

Legal experts and former prosecutors say the charge is possible if there is sufficient evidence the slaying was motivated by racial prejudice, and was not simply a fight that spiraled out of control.

So far, only one such clue has surfaced publicly against George Zimmerman, 28, the neighbourhood watch captain who fatally shot the black 17-year-old on Feb. 26 in the central Florida town of Sanford.

"If that was a racial epithet that preceded the attack on Trayvon Martin, we definitely have a hate crime."

—Donald Tibbs, a Drexel University law professor

On one of his emergency calls to police that night, Zimmerman muttered something under his breath that some listeners say sounds like a racial slur.

"It sounds pretty obvious to me," said Donald Tibbs, a Drexel University law professor who has closely studied race, civil rights and criminal procedure. "If that was a racial epithet that preceded the attack on Trayvon Martin, we definitely have a hate crime."

However, others say the recording is not clear enough to be certain of what Zimmerman said. And many experts say more evidence would be needed that he harboured racial prejudice against black people and went after Martin for that reason alone.

220-zimmerman-fla-02352019

George Zimmerman, a Sandford, Fla., neighbourhood watch captain, insists he shot unarmed teenager Trayvon Martin in self-defence. (Orange County Jail via Miami Herald/Associated Press)

There had previously been burglaries in the complex committed by young black males, possibly heightening Zimmerman's suspicions when he spotted Martin.

"They are going to have to show he was specifically targeting this individual based on his race, creed, colour, et cetera," said David S. Weinstein, a former federal prosecutor in Miami. "Not that he was chasing somebody down and got in a confrontation that may or may not have been based on that."

The case's racial overtones have sparked a national debate with civil rights groups saying the shooting was unjustified and demanding the arrest of Zimmerman.

Zimmerman's parents, in a letter to a local newspaper, insisted their son is not a racist, and several black residents of the neighbourhood where Martin was shot have only good things to say about him. Zimmerman's father is white and his mother is Hispanic.

Controversial law

So far, Zimmerman has not been charged with any crime and is claiming self-defence under Florida's "stand your ground" law, which eliminated a person's duty to retreat when threatened with serious bodily harm or death. He claims Martin attacked him as he was walking back to his truck, according to police.

"He's not a racist," attorney Craig Sonner said about his client. "The incident that transpired is not racially motivated or a hate crime in any way."

Those "stand your ground" laws, in place in about two dozen states, have come under increasing scrutiny since the shooting.

"He's not a racist ... the incident that transpired is not racially motivated or a hate crime in any way." — Craig Sonner, George Zimmerman's attorney

U.S. Sen. Charles Schumer on Sunday sent a letter to U.S. Attorney General Eric Holder asking for a federal investigation into whether killings are going unprosecuted because the laws put too much of a burden on local authorities.

Preaching before hundreds at Macedonia Missionary Baptist Church in Eatonville, Fla., about 32 kilometres from the site of the Sanford shooting, Rev. Jesse Jackson said the stand your ground law is "an incentive for vigilantes to kill people" and referred to Martin as a "martyr."

"How do we turn pain into power?" the civil rights activist asked a standing-room only congregation on Sunday. "How do we go from a moment to a movement that curries favour?"

Martin's parents and hundreds of supporters say Zimmerman should have been immediately arrested and charged with the youth's killing, but local police say they have little evidence to disprove his self-defence claim.

A grand jury will be convened April 10 to consider whether to bring state charges, which could include second-degree murder or manslaughter.

After receiving a no-confidence vote from the city commission, Police Chief Bill Lee announced last week he was temporarily stepping aside from his post.

The city manager, Norton Bonaparte Jr., said officials want the case to be resolved fairly.  

Potential life sentence

Civil rights activist the Rev. Al Sharpton, who has been appearing at rallies with Martin's parents to call for an arrest, said the Justice Department should investigate the case as a hate crime.

"Any time you have a pattern of engagement based on someone's having a particular group in mind, that qualifies for hate crime inquiry," Sharpton told The Associated Press.

The Justice Department's civil rights division and the FBI are conducting their own probe in the case, and a federal hate crimes charge could come out of that no matter what state authorities do.

"Any time you have a pattern of engagement based on someone's having a particular group in mind, that qualifies for hate crime inquiry."—Civil rights activist Rev. Al Sharpton

The hate crimes law carries a potential life prison sentence when a death is involved.

Tibbs said one key is determining whether Martin's race alone was the reason Zimmerman decided to follow him in his vehicle. Martin, who was from Miami, was staying in the neighbourhood with his father and father's fiancée and was returning from a convenience store with a bag of Skittles candy and a can of iced tea when the confrontation took place.

He was not armed.

"He was not suspicious. What makes him suspicious in the moment is the fact that he was black. If Trayvon Martin was white, would any of this have happened?" Tibbs said.