(CNN) -- 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 a previously released video, which had a grainy quality.
The 28-year-old neighborhood watch volunteer has said he killed the unarmed 17-year-old 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 Martin, 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 certain 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, Crump and the Martin family pressed for a federal investigation into the decision to not arrest Zimmerman, saying someone from the state attorney's office ignored a Sanford detective's advice to arrest Zimmerman.
In a 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 Martin 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.
ABC News has reported that the lead homicide investigator, Chris Serino, filed an affidavit pushing for charges the night of the killing, but was overruled by the state attorney's office.
Neither Sanford police nor prosecutors have confirmed the existence of such an affidavit. 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
"I am outraged by the outright lies contained in the letter by Benjamin Crump," Wolfinger said in a statement Monday.
"I have been encouraging those spreading the irresponsible rhetoric to stop and allow State Attorney Angela Corey to complete her work," he added. "Another falsehood distributed to the media does nothing to forward that process."
Crump responded to Wolfinger's statement, saying he is outraged by the decision to not arrest Zimmerman and said the Martin family simply wants answers.
"Why did they overrule? Why is George Zimmerman free? Who made this decision? This is is something they've asked repeatedly," the attorney said. "Why wasn't he arrested? That's at the crux of this matter."
Crump said no one is saying Zimmerman can't make a self-defense claim in a court of law.
"All we're saying is that he should have been arrested. If that was Trayvon Martin who was accused of pulling the trigger, he would have been arrested right there on the spot. We only want equal justice and 'fair and impartial' to be applied across the board."
The next major movement in the case likely will come from Corey's office, which could charge Zimmerman, clear him or send the case to a grand jury.
CNN's Deborah Feyerick, Jason Basso, Wolf Blitzer and Vivian Kuo contributed to this report.