The mother of a Miami boy who was fatally shot in central Florida said Friday she cannot understand why the shooter has not been arrested. "As a mother, my heart is broken," Sybrina Fulton said. Her son Trayvon Martin, 17, a junior at Dr. Michael Krop Senior High School in Miami, was in Sanford visiting his father when George Zimmerman allegedly shot him on Feb. 26. Police said Zimmerman told them that he shot Martin in self-defense. Sanford Police Chief Bill Lee Jr. said that evidence and testimony may corroborate Zimmerman's story, because he had a bloody nose and had grass clippings on the back of his shirt, like he had been in a defensive posture.