PHILADELPHIA -- The mother of slain teenager Trayvon Martin told a National Urban League gathering Friday to use her tragedy to stop the same thing from happening to another child.

Sybrina Fulton also blamed Florida's "stand your ground" law for allowing her son's killer to go free. She issued a call to action against such self-defense laws and told the Philadelphia audience that she believes God is using her and her family to make a difference.

Fulton said she was heartened that her message is getting through when, on the way to the civil rights organization's annual conference, she handed a card bearing her son's photograph to a driver and he told her, "That's my son, too."

"So my message is coming across, and it's coming across the right way," Fulton said.

The message, she said, is to use her son's story to prevent other children from dying the same way. "My message to you is please use my story, please use my tragedy, please use my broken heart to say to yourself, 'We cannot let this happen to anybody else's child,' " she said.

Fulton supports an investigation into whether her son's killer could be charged under federal civil rights laws and she attacked Florida's "stand your ground" as having "prevented the person who shot and killed my son to be held accountable and to pay for his awful crime."

A jury acquitted George Zimmerman of all criminal counts in the slaying of the unarmed 17-year-old. Martin was shot and killed during a confrontation with Zimmerman that has sparked a national debate about "stand your ground" laws in Florida and other states.

Earlier yesterday, a juror told ABC's "Good Morning America" that she feels like she let down Martin's parents. Juror B29 said in the taped interview that the panel couldn't find enough evidence under Florida law to convict the neighborhood watch volunteer in Martin's death last year.

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