ATLANTA (CBS News) - The parents of an unarmed teenage boy who was shot and killed in Sanford, Florida last month are now calling on the FBI to get involved in the investigation.

Trayvon Martin's parents have formally asked the FBI to investigate this case, because they believe local police and prosecutors have not done their job. The admitted gunman remains free - and this case has growing overtones of race and injustice.

Photo Gallery:FL teen killed by neighborhood watch volunteer

Martin's parents say recently-released 911 calls prove the shooter, a neighborhood watch volunteer, was not acting in self-defense, as he has claimed.

Hundreds of Martin's supporters marched near Orlando on Sunday, outraged that three weeks after the shooting death of the 17-year-old, police have not arrested the gunman.

Martin was visiting relatives on February 26, walking, unarmed, in their subdivision on his way back from a 7-11 store. It was 7 p.m.

George Zimmerman, a community watch member, thought Martin looked suspicious and called police.

"He's got his hand in his waistband and he's a black male," Zimmerman is heard telling the dispatcher.

After Zimmerman confirms to the dispatcher that he is following Martin, the emergency operator tells him, "we don't need you to do that."

But Zimmerman ignored the dispatcher. He caught up with Martin. They fought.

Two minutes after Zimmerman was told police would handle it, seven neighbors began calling 911 about hearing screams, and gun fire.

Martin was killed, shot once in the chest. Zimmerman told police he fired his 9-millimeter pistol in self-defense, and he has a lawful concealed weapons permit. But all Martin was carrying was his cell phone, a can of ice tea and a bag of skittles.

Police in Sanford Florida released the 911 recordings last Friday after pressure from Martin's family and supporters.

Tracy Martin, the teen's father, says his son was murdered, and can't understand why Zimmerman is still free.

"They're treating this as my son is the perp," says the father. "My son is the victim here. There's no reason why he should have been in that situation, none whatsoever."

The bereaved father dismisses the claim of self-defense.

"It can't be self-defense," says Tracy Martin. "What was he gonna do, attack him with a bag of Skittles?"

Florida law has a "stand your ground law," which allows wide latitude in using deadly force if someone feels reasonably threatened.

Zimmerman's not giving interviews. State prosecutors will now decide whether he met the legal standard for the state's self-defense law, or whether he should be arrested.

The problem, explains CBS News legal analyst Jack Ford, is that only two people are known to have witnessed the incident - Zimmerman, who insists he was attacked and was only defending himself, and Martin, who was killed.

Thus, the most important thing for lawyers on both sides to do, will be to try and find anybody else who can provide some account of the altercation which ended in the fatal shooting of an unarmed teenager.

Photo Gallery:Trayvon Martin photos

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