MIAMI (AP) -- Legal experts say the U.S. Justice Department could bring a hate crime charge against the shooter in the killing of a black Florida teenager.

But that will require a finding that neighborhood watch captain George Zimmerman shot 17-year-old Trayvon Martin because of racial bias.

Drexel University law professor Donald Tibbs thinks Zimmerman's bias is obvious. He thinks a 911 tape shows Zimmerman muttering a racial slur just before last month's shooting.

But others say that the recording isn't conclusive and that there is nothing definitive showing the 28-year-old Zimmerman acted because of prejudice. They say he might have followed Martin because the neighborhood had been hit by a string of burglaries committed by young black males.

Martin was heading back to the home of his father's fiance when he was shot.