SANFORD, Fla. -- FBI agents on Monday were questioning potential witnesses in the Trayvon Martin shooting, confirming to NBC News that the agency had begun a "parallel investigation" that focuses on whether the teen's civil rights were violated.
Agents are seeking information on George Zimmerman's background and whether he was racially motivated when he pursued Martin after calling a 911 police dispatcher about his presence in the community, an FBI official told NBC.
The agents were at the Retreat at Twin Lakes, where Martin was shot dead by Zimmerman, the gated community's neighborhood watch captain.
The state of Florida is conducting its own investigation in the shooting and that one is being overseen by a special prosecutor.
Although the Justice Department two weeks ago publicly announced it would examine potential civil rights violations, the arrival of bureau agents represents a new phase in inquiries into the case.
A senior law enforcement official confirmed that one potential piece of evidence is records of Zimmerman's prior 911 calls to police dispatchers.
The call sheets show that five of seven phone calls Zimmerman had made since last August involved what he viewed as suspicious activity by young men identified as "black males." But the call sheets do not indicate whether Zimmerman was asked about the race of the suspects or volunteered that information.
Zimmerman, who has not come forward to talk about the shooting, acknowledged to police that he shot Martin but insists it was self-defense.
The incident has sparked outrage in many quarters because Martin, who was black, was unarmed.
Craig Sonner, Zimmerman's lawyer, has insisted that his client, who has a white father and Hispanic mother, is not racist. "This was not a case of racial profiling," he told NBC News on Sunday. "George Zimmerman is not a racist."
Sonner told Reuters on Monday that Zimmerman would turn himself in to police should he be charged.
Also Monday, lawyer's for Martin's parents said they would ask the Justice Department to investigate the local prosecutor's interactions with police handling the probe into their son's death.
The Justice Department initiated a query on March 19, but the family wants them to look into possible interference by State Attorney Norm Wolfinger's office with the Sanford police investigation of Martin’s Feb. 26 death, NBCMiami.com reported.
The Sanford Police Department requested an arrest warrant from Wolfinger’s office in the shooting, but the state attorney's office held off until the case could be reviewed further, according to NBCMiami.com.
Experts: Screams on 911 call not Zimmerman
Video shows Zimmerman after shooting
Two audio experts who have analyzed the 911 tapes from the fatal evening have determined that Zimmerman was not the person heard crying for help.
"The tests concluded that it's not the voice of Mr. Zimmerman," said Tom Owen, of Owen Forensic Services LLC and chair emeritus for the American Board of Recorded Evidence.
Asked if he thought such tests would be admissible in court, Owen said "yes" and noted he had recently used similar testing in testimony at a Connecticut murder case that involved a 911 call.
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