SANFORD, Fla. - Two new pieces of evidence appear to lend credence to opposing versions of what happened the night Trayvon Martin died.
Enhanced surveillance images of George Zimmerman, the man who admitted shooting the teen but claimed self-defense, appear to show a bump, mark or injury on the back of his head.
But two forensic audio experts said a voice crying for help on a 911 call does not appear to be Zimmerman's voice, despite claims to the contrary by his family.
Police in Sanford, Florida, released a new, higher-resolution surveillance video of Zimmerman entering the police station the night of Martin's death on February 26. The sharper footage shows an apparent bump, mark or injury on Zimmerman's head more clearly than previously released video, which had a grainy quality.
The 28-year-old neighborhood watch volunteer has said he killed Martin in self-defense, saying the teen punched him and slammed his head into a sidewalk before the shooting, according to family members and police.
Benjamin Crump, an attorney for the Martin family, said Monday night that it's up to a court of law to determine whether the enhanced video supports Zimmerman's claim of self-defense.
But he added, "Is that enough to justify deadly use of force to kill an unarmed teen?"
He also said Martin would not be dead if Zimmerman had not gotten out of his car and confronted the teen.
Martin's family and supporters say Zimmerman, who is Hispanic, profiled the boy, who was black, as "suspicious" and ignored a police dispatcher's request that he not follow him. The 17-year-old had a bag of Skittles candy and an iced tea at the time of his death.
Another point of debate is a 911 call made the night of Martin's death.
Zimmerman has said he was yelling for help, according to his family members and his account to authorities -- as first reported by the Orlando Sentinel and later confirmed by Sanford police.
But Martin's relatives, including his cousin Ronquavis Fulton, have said they are sure the voice heard on the 911 call is that of Trayvon Martin.
Audio experts Tom Owen and Ed Primeau, who analyzed the recordings for the Sentinel using different techniques, said they don't believe it is Zimmerman who is heard yelling in the background of one 911 call. They compared the screams with Zimmerman's voice, as recorded in a 911 call he made minutes earlier describing a "suspicious" black male.
"There's a huge chance that this is not Zimmerman's voice," said Primeau, a longtime audio engineer who is listed as an expert in recorded evidence by the American College of Forensic Examiners International.
"After 28 years of doing this, I would put my reputation on the line and say this is not George Zimmerman screaming."
Owen, a forensic audio analyst and chairman emeritus of the American Board of Recorded Evidence, also said he does not believe the screams came from Zimmerman.
He cited software that is widely used in Europe and has become recently accepted in the United States that examines characteristics like pitch and the space between spoken words to analyze voices.
Using it, he found a 48% likelihood the voice is Zimmerman's. At least 60% is necessary to feel confident two samples are from the same source, he told CNN on Monday -- meaning it's unlikely it was Zimmerman who can be heard yelling.
The experts, both of whom said they have testified in cases involving audio analysis, stressed they cannot say who was screaming.
Meanwhile, the Martin family's attorney continued pressing for a federal investigation into the decision not to arrest Zimmerman, saying someone from the state attorney's office ignored a Sanford detective's advice to arrest Zimmerman on suspicion of manslaughter.
In the letter delivered to the U.S. Justice Department on Monday, the Martin family said the Sanford police detective "filed an affidavit stating that he did not find Zimmerman's statements credible in light of the circumstances and facts surrounding the shooting."
The family said Sanford Police Chief Bill Lee and State Attorney Norm Wolfinger met the night of the shooting and disregarded the detective's advice, allowing Zimmerman to remain free.
Neither Sanford police nor prosecutors have confirmed the existence of such an affidavit, which ABC News first reported. Sanford officials and special prosecutor Angela Corey's office declined comment.
But Wolfinger, who stepped aside in the case last month, vehemently denied that any "such meeting or communication occurred" between him and Lee.
"I have been encouraging those spreading the irresponsible rhetoric to stop and allow State Attorney Angela Corey to complete her work," Wolfinger said in a statement Monday. "Another falsehood distributed to the media does nothing to forward that process."
Crump said the Martin family simply wants answers.
"Why did they overrule? Why is George Zimmerman free? Who made this decision? This is something they've asked repeatedly," the attorney said. "Why wasn't