A Justice Department spokesman said the agency will review the Martin family letter.
State prosecutors are expected to go before a Seminole County grand jury on April 10 to determine what, if any, files should be charged.
An analysis of a 911 call done over the weekend by the Orlando Sentinel determined that screams for help overhead on a 911 came from Martin, although Zimmerman's family insist they recognize his voice in the screams.
Two evidence experts consulted by the Sentinel found the voice heard in a 911 call placed by a woman in a home near where the shooting occurred was only a 48 percent match to Zimmerman's voice. One of the experts, Tom Owen, told the Sentinel to reach a positive match he would expect higher than 90 percent.
"As a result of that, you can say with reasonable scientific certainty that it's not Zimmerman," Owen told the paper.
Owen,the chair emeritus at the American Board of Record Evidence, was not able to determine if the voice was that of Martin, the Sentinel reports, because he did not have audio of the teen's voice to compare to the shouts for help in the 911 call.