MIAMI  - More than one million people have signed an online petition calling for criminal charges against a crime-watch volunteer who fatally shot an unarmed black teenager in a gated Florida community, organizers said Thursday. The activist group said their campaign was seeking justice for 17-year old Trayvon Martin, killed after being shot last month while walking in the gated community in the central Florida city of Sanford. The shooter, neighborhood watch captain George Zimmerman, was released after he claimed he acted in self-defense.

Police have said they believe he is immune from being prosecuted in the case thanks to the city’s “Stand Your Ground” law, which allows residents in the state to use lethal force in self-defense.

Zimmerman told police that the two got into a fight and the teenager was the instigator of the violence.

Some residents of Sanford, and the larger African American community across the United States, said the case is the latest example of the racial profiling and unjust treatment faced by blacks by the country’s criminal justice system.

Organizers of the online petition drive, who included the dead teen’s grieving parents, said they have been registering new signatures at the rate of 50,000 per hour.

“I’m a normal mother,” said Sybrina Fulton. “My heart is broken. I keep thinking he is going to walk in through the door and the nightmare is going to end. I just want justice for my son,” she said.

Trayvon’s father, Tracy Martin, said the family was not “looking for revenge, we’re looking for justice — the same justice anyone would expect if their son were shot and killed for no reason.”

The US Justice Department said earlier this week that its Civil Rights Division plans to “conduct a thorough and independent review of all of the evidence.” The division examines cases in which discrimination on the basis of race, sex, disability, religion, and national origin are suspected.

The FBI is also investigating the racially charged killing.

Civil rights advocate Al Sharpton, meanwhile, was planning a protest march in Sanford later Thursday to apply additional pressure on official to prosecute Zimmerman.