The Florida special prosecutor investigating the Trayvon Martin shooting is bringing in independent voice analysis experts to enhance 911 tapes to learn more about the actions of George Zimmerman, the neighborhood watch volunteer who shot and killed the teen.

Angela B. Corey, who was appointed by Gov. Rick Scott (R) to investigate the shooting, said the experts will examine two potentially crucial aspects of the eight calls made to 911: a racial epithet Zimmerman might have muttered while talking to a dispatcher minutes before shooting the unarmed 17-year-old on Feb. 26, and the screams in the background of calls that residents made to police during the incident.

Screams can be heard in at least one call made to police before officers arrived at the scene in Sanford, Fla. Martin’s parents say the voice is their son yelling for help; a friend of Zimmerman’s has said it is Zimmerman screaming.

“The exact words and whose voice is whose will be the critical issues,” Corey said in an interview.

The Justice Department is also investigating the incident. Zimmerman’s 911 call, made moments before the shooting, is central to a case the federal government could bring under federal civil rights laws, Justice officials said.

Allegations have been made that Zimmerman used a racial epithet on the call, but the tape is unclear. If an epithet can be heard clearly on an enhanced recording, it could be evidence of a hate crime, the officials said.

Zimmerman told police that in a struggle for his gun, he shot Martin. The teen was shot in the chest at close range.

A man who spoke on the condition of anonymity told ABC News on Tuesday that he saw a man in a red sweater on the ground and that someone was beating him up. Zimmerman was wearing red that night. The eyewitness said: “The guy on the bottom was yelling, ‘Help, help!’ I told him to stop, and I was calling 911.” Another eyewitness, 13-year-old Austin Brown also told ABC that he saw a man on the ground wearing red.

On the night of the shooting, the Sanford police department’s lead homicide investigator in the case had recommended that Zimmerman be charged with manslaughter, but he was overruled by a state’s attorney, who said there was not enough evidence in light of the state’s “Stand Your Ground” law, according to ABC News.

The FBI and the Justice Department are helping Florida investigators with resources but letting state prosecutors take the lead, law enforcement officials said. A Seminole County grand jury is scheduled for April 10.

Corey said a grand jury might not be necessary, suggesting that if she finds enough evidence to file charges, she could do so.

Martin’s family flew to Washington on Tuesday and attended a Capitol Hill briefing about racial profiling and state “Stand Your Ground” laws.

“Mr. Zimmerman should be arrested immediately for his own safety,” said Rep. Frederica S. Wilson (D-Fla.). Among those in the packed hearing room was Ernest Green, one of the Little Rock Nine, who in 1957 were the first black students to attend Little Rock Central High School.

Benjamin Crump, the attorney for Martin’s parents, also painted a fuller picture Tuesday of the teen. He said Martin was suspended from his Miami high school three times over the past year.

Martin was staying with his father’s fiancee in a gated community in Sanford because he had been suspended for 10 days in February after an empty baggie containing marijuana residue was found in his book bag, Crump said.

According to the Miami Herald, a marijuana pipe was listed, along with the baggie, in a school report on the incident. And Martin was suspended four months earlier for writing graffiti on school grounds, the Herald reported.

A school police guard said that after the graffiti incident, he went through Martin’s backpack, the Herald reported. The officer said he found 12 pieces of jewelry, including wedding bands, diamond earrings and a watch. He said he also found a screwdriver, which he described as a “burglary tool.”

Martin was suspended for the graffiti, according to the report. The school police confiscated the jewelry and sent photos to Miami-Dade police detectives.

Ryan Julison, a spokesman for the Martin family, said the family knew about the graffiti but not the jewelry incident. “The fact that this private, sensitive information about a dead teenager is being leaked to the media is reprehensible,” Julison said.