Federal prosecutors and the FBI have opened an investigation into the killing of an unarmed teen by a neighborhood watch volunteer in Florida.

The death drew protesters Monday to the courthouse in Seminole County, north of Orlando, to demand justice for Trayvon Martin, the 17-year-old who was shot and killed last month while walking back to his father's fiancee's house in Sanford.

The African-American teen's parents said Monday that they believed race was a factor in their son's death, and the Congressional Black Caucus had called for a federal investigation, saying local police have shown "blatant disregard for justice."