Parents' fight for justice for their son Trayvon Martin

Tracy Martin speaks softly, but his words carry a terrible weight. His teenage son Trayvon has been shot dead, and the man who did it walks free.

Tracy and Trayvon were settling down together one night last month to watch a basketball game on the TV at home. Trayvon popped out to the convenience store to buy some sweets. He didn't make it home. On the way back he was followed by a neighbourhood watch guard, George Zimmerman.

Mr Zimmerman thought Trayvon was acting suspiciously, and rang the police to tell them so. The police told him that he didn't need to follow the teenager, but he did anyway.

There was an altercation, which ended with the teenager lying face down on the ground, with a bullet in his chest.

When the police arrived Mr Zimmerman told them he had acted in self-defence. They believed him. No arrest was made.

In Florida, the law says that if you are in fear of 'great bodily harm', you can respond with lethal force, and you will not be prosecuted. One Floridian state senator says that the so-called 'Stand Your Ground' law has increased killing in the state by 250 percent. It sounds like the Wild West.

Tracy has a handle on the law in Florida, but he says it would have been applied differently if his boy's skin colour had been different. If the police had found a white boy lying on the sidewalk with a gunshot wound, he says, they would have somebody in custody.

When I met Tracy in New York City he said that if it had been his son who had done the shooting, he wouldn't be doing TV interviews with me; he'd be talking to his son, through a plate of glass in the county jail.

Tracy and Trayvon's mother, Sabrina, are, as they say in America, just a regular people. Tracy is a truck driver and Sabrina works for the local authority. They cut distinguished figures as they stand together at a demonstration about their son's death.

"People are outraged that another black teenager has been killed and nothing is being done about it", says Tracy.

Sabrina, on the other hand, seems very concerned that the colour of her family's skin doesn't colour what is for her the main issue: the apprehension of her son's killer.

"This isn't about a black or white thing", she says, "It's about a right and wrong thing".

Four years ago, another 'race story' came out of the United States, and it was a really good one. The election of a black president. That night in Chicago, Barack Obama told the world that change had come to America.

When I repeated his words to a young black couple at a Trayvon protest in Manhattan, I got an angry response. "You think electing a black president is enough? Racism is institutionalised here", they told me.

In Florida, the police chief has resigned and there are now almost too many investigations into Trayvon's death to count: from the FBI to a state grand jury.

The system wants to crack down on any suggestion of racism as soon as it finds it. The disturbing thought is that racism still runs through the system itself.