Trayvon Martin's parents will ask the U.S. Justice Department to review a local Florida prosecutor's interactions with police investigating his killing, their lawyer said Sunday.

Meanwhile, two forensic voice identification experts said the voice heard crying for help on a 911 call just before Martin was shot to death Feb. 26 was not that of his killer, George Zimmerman, who claims self-defense.

The Justice Department launched an investigation into Martin's death on March 19, but the family is now asking it to look for possible interference by State's Attorney Norm Wolfinger's office with Sanford, Fla., Police Detective Chris Serino, attorney Ben Crump said.

The Martin family will send a formal request to the Justice Department Monday, Crump told CNN Sunday.

Martin, 17, was fatally shot by neighborhood watch volunteer Zimmerman, 28, after Zimmerman called police to report him as a suspicious person on the evening of Feb. 26.

As for the 911 tapes, the tests concluded that it's not the voice of Mr. Zimmerman, Tom Owen, of Owen Forensic Services LLC and chairman emeritus for the American Board of Recorded Evidence, told MSNBC.

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 911 calls.

The conclusions of Owen and another audio expert were first reported by the Orlando Sentinel on Saturday.

Zimmerman told police that he screamed for help during his confrontation with Martin.

The 911 call, reposted in this YouTube clip, came on the night of Feb. 26 from a woman who reported someone crying out for help in her gated community in Sanford, Fla. In the recording of her phone call, panicked cries and a gunshot are heard.

The Sentinel said it had contacted the two audio experts.

Owen told the newspaper he used software called Easy Voice Biometrics to compare Zimmerman's voice to the 911 call screams.

I've run it against 300 voices and it was better than 99 percent in all cases, he told MSNBC when asked about its accuracy.

Owen told the newspaper that the software compared the screams to Zimmerman's voice and returned a 48 percent match. He said he would expect a match of higher than 90 percent, considering the quality of the audio.

As a result of that, you can say with reasonable scientific certainty that it's not Zimmerman, Owen told the Sentinel. 

But he also said he could not confirm the voice as Trayvon's, because he didn't have a sample of the teen's voice.

The Sentinel said that Ed Primeau, a Michigan-based audio engineer and forensics expert, used audio enhancement and human analysis and came to the same conclusion.

Zimmerman says he killed Martin in self-defense after the teen punched him and slammed his head on the sidewalk, according to an Orlando Sentinel report that was later confirmed by Sanford police.

Police cited Florida's stand your ground law, which allows people to use deadly force anywhere they feel a reasonable threat of death or serious injury, for not immediately arresting Zimmerman.

Authorities have said Zimmerman was not immediately charged because there were no grounds, at the outset, to disprove his account that he'd acted to protect himself. Martin's family and supporters say Zimmerman, who is Hispanic, racially profiled the teen, who was black, and ignored a police dispatcher's directive not to follow him.

The killing sparked nationwide protests, including a march Saturday by civil rights luminaries, carrying Justice for Trayvon signs in a marching to the Sanford police headquarters.

The Sanford police department has come under intense scrutiny for its actions following the shooting, and protesters renewed their call for the firing of police Chief Bill Lee, who stepped aside temporarily this month amid criticism.

The local prosecutor also stepped aside. Gov. Rick Scott appointed a special investigator to decide if Zimmerman should be charged, cleared or if the case should be sent to the grand jury.