In the latest update in the tragic Trayvon Martin shooting, the US Department of Justice could bring a hate crime charge against 28-year-old George Zimmerman, the self-appointed neighborhood watch captain who shot and killed the teen in a gated central Florida community.

The charge would be based on evidence that the incident was motivated by racial bias, reports the Associated Press. Zimmerman, who is half White and Hispanic, muttered what some are calling a racial slur in his 911 call to police shortly before he shot Martin. Listeners of the 911 call claim that Zimmerman whispered “f**king coons” as he described Martin’s location to the 911 operator.

Others, however, claim that the recording is not clear enough to discern Zimmerman’s whisper. Experts say that more evidence would be needed that he harbored racial prejudice against African-Americans and pursued Martin for that reason alone.

In the weeks following Martin’s death, a national uproar has resulted from Zimmerman not being arrested or charged with killing Martin. Zimmerman is claiming self-defense under Florida’s “Stand Your Ground” law, which grants immunity to anyone who uses deadly force if he can reasonably claim he was defending himself. He claims Martin attacked him as he was walking back to his truck, according to police.