SANFORD, Fla. -- The U.S. Department of Justice's Civil Rights Division and the FBI are investigating the killing of a Miami teen by a neighborhood watch volunteer.

College students from across Florida are holding rallies to demand the arrest of George Zimmerman. He's the man who police say shot and killed 17-year-old Trayvon Martin.

Last month, Zimmerman called 911 and said someone suspicious was walking through his neighborhood.

Zimmerman said he shot the teen in self defense, but some eyewitnesses say that's not true.

Martin's family just wants justice.

"The whole situation is a nightmare. I don't understand why this man has not been arrested,” said Martin’s mother. “There is a hole in my heart, he was my baby. I just don't understand how someone can just take his life."