So far, only one such clue has surfaced publicly against 28-year-old George Zimmerman, the neighborhood watch captain who fatally shot the 17-year-old Martin on Feb. 26 in the central Florida town of Sanford. On one of his 911 calls to police that night, Zimmerman muttered something under his breath that some listeners say sounds like a racial slur. Zimmerman's father is white, and his mother is Hispanic.
"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."
Others, however, say the recording is not clear enough to determine what Zimmerman actually said. And many experts say more evidence would be needed that he harbored racial prejudice against black people and went after Martin for that reason alone. 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, color, 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."
Zimmerman's parents, in a letter to a local newspaper, insisted their son is not a racist, and several black residents of the neighborhood where Martin was shot have only good things to say about Zimmerman. Zimmerman has not been charged with any crime and is claiming self-defense 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. U.S. Sen. Charles Schumer, D-N.Y., 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.
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-defense 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.
Civil rights activist 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. 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 neighborhood with his father and father's fiancTe and was returning from a convenience store with Skittles 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.
If Zimmerman were a police officer or a government official, he could be prosecuted by the Justice Department for using his official authority to violate Martin's civil rights. That was the case made against Los Angeles police officers who had been acquitted in state court of beating Rodney King, which sparked huge riots. Two of the four officers were eventually convicted of federal civil rights violations.
But Zimmerman was a volunteer watch captain, and even though he had a permit to carry his Kel Tek 9mm semiautomatic handgun, he didn't have any official law enforcement or government authority.
Another possibility is an investigation of the Sanford Police Department itself, including questions about whether any evidence was destroyed or covered up, or whether there has been a pattern of problems involving black people. City officials insist they did an appropriate and thorough investigation, but if such violations occurred federal prosecutors could bring civil rights conspiracy charges against anyone responsible. Bonaparte did acknowledge last week that the police department has had issues with the city's African-American residents.
For instance, in 2010, it took a month for investigators to arrest and charge the son of a police lieutenant who was accused of knocking out a homeless black man. The attack was captured on video.
"This police department, how they've handled this case and how we are hearing of other cases, needs a thorough review by the Justice Department," Sharpton said.
Ultimately, much depends on the results of the state grand jury investigation. If Zimmerman ultimately is charged in Martin's death, the Justice Department may not bring its own separate case depending on the outcome of any trial.
The mere presence of federal investigators could ensure a more thorough probe, said University of Florida law professor Michael Siegel.
"A lot of times when these things are getting potentially explosive, they want to step in and say, `We're looking over the shoulder of the locals here,"' said Siegel, a former federal prosecutor. "That often helps calm the public down. It does put some pressure on the local law enforcement to take a second look and do it right."