Miami ? The U.S. Justice Department could bring a hate crime charge against the shooter in the killing of black Florida teenager Trayvon Martin if there is sufficient evidence the slaying was motivated by racial bias and not simply a fight that spiraled out of control, legal experts and former prosecutors say.

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.