Media playback is unsupported on your device Media caption Tensions are high in Sanford, Florida as George Zimmerman's trial opens, says the BBC's Mark Mardell

Cecil Smith, the new police chief in Sanford, Florida, is full of energy, bounding down the road of single-storey, wooden houses which are nestled among the dark, green lushness of Florida's flora.

He is on a mission to "reconnect with the community". The black residents look on curiously at this African-American officer going out of his way to be nice.

One man, carrying a six-pack of vodka coolers, tries to show him his driving licence, to prove he is over 21.

"No need, no need," says Chief Smith.

He goes up to a group of men having a barbecue at the front of their house. There is an odd notice on it saying that police can ask people to disperse if they are making noise.

The chief shakes hands all round and then clasps one of the men tight, clapping him on the back. The man's expression suggests it is not every day he is hugged by a cop.

'Changing perceptions'

There is not a lot of trust in the police in this part of town, after the shooting of an unarmed black teenager, Trayvon Martin, by a volunteer neighbourhood watchman, George Zimmerman.

The police took weeks to arrest him, accepting his story that he thought the teenager was acting suspiciously, and had attacked him.

He was eventually charged with murder and the trial starts today.

It is going to be a difficult time for the town. One activist explained to me why the case became a cause throughout the city, and throughout out the nation.

"Zimmerman walked in the front door of the police station and out the back," the activist says.

"It said to black people here that black boys can get killed and all you've got to say is, 'I was standing my ground,' and you can walk away."

I want to know if the new chief of police agrees that young black men are assumed to be up to no good.

"I am not going to stand here and say every black kid is a criminal. I have two black sons," Chief Smith says.

So do black teenagers get a raw deal?

"It does happen. Our biases, sometimes people see the colour before they know the person and that is the perception that we are trying to change within the city of Sanford," he adds.

That is pretty diplomatic. The guy on the front porch with the barbeque is not.

"Zimmerman should hang," the neighbour says. "They just let him go to show they can kill a black boy."

'Race card'

Many white people despair of this attitude. Few will say so on camera or into a recorder, but there are some who say privately that "they" should "get over it".

"They" and "them" are words you hear a lot around town.

Image copyright AP Image caption George Zimmerman (left) and Trayvon Martin

Sanford is a city that is mixed, but not very mingled. Black and white residents both talk of "their" areas and "their" schools.

It makes life awkward for neighbourhood watch, whose job is to spot people who seem out of place, perhaps up to no good.

George Zimmerman was a self-appointed neighbourhood watch captain, on patrol when he shot Trayvon Martin. And Wanda Chandler feels it was a tragedy.

She is deeply involved in neighbourhood watch and takes me through the back alleys, rich with palms and trees draped with Spanish moss, telling me that such short cuts make life easy for burglars.

She says it has been awkward since the shooting.

"We don't want anyone to think we are racially profiling anyone," Ms Chandler says.

"It's a fact, not a perception, that most of residential burglaries are caused by young teens, young black men.

"It makes it difficult even calling in a crime. You don't want to be prejudicial, but it's just a description, the colour of someone's skin."

Ms Chandler feels the case has been deliberately made political by people with an agenda, who are not bothered by the facts of the case - or facts in general.

She says people need to get educated.

"You can't always play the race card," Ms Chandler says. "Things just happen.

"But when people call civil rights groups into a small town like this, that's predominantly white, it can make tensions worse - you go back to a mentality of the 1960s.

"People need to get along, no matter what colour they are. People need to let go, they believe we still live in a society where people are racially profiled, racially this, racially that - and it's bad," she says.

"People need to step up to the plate and realise that things happen - not because of the colour of your skin. Things just happen."

A third dynamic

Standing in front of the flat, slab-like memorial to Trayvon Martin in Goldsboro, Francis Oliver could not disagree more.

She runs the town's little museum, which has a heavy focus on the struggle for civil rights.

The town is interesting: one of the first black cities to be incorporated in Florida with its own mayor and officers.

But it lost all that when it became part of Sanford in 1911.

Media playback is unsupported on your device Media caption The racially charged case has divided the US

Inside the museum there are the separate drinking fountains of the Jim Crow era, marked "black" and "white".

There are booklets from further back still, including one that contains advice on how to break the spirit of a slave. I feel slightly sick after flicking through them.

Ms Oliver says the fact that the memorial to Martin is here, rather than where it first stood, speaks volumes.

"We're eight to 10 miles away from it," she says. "It was right at the gates, where he was shot, in front of the apartment buildings where he was shot.

"But some taxpayers and some of Zimmerman's friends did not want it there, so they complained to the city and the city moved it. They moved it five times," Ms Oliver says.

"But it's here now."

For her it is a symbol of the way race relations have become worse since the shooting.

"I think it was terrible - to me it was selfish, racial. I thought if it had been a white boy it wouldn't be done that way, and it was immature."

But Ms Oliver thinks some of the white anger about the case is political, too.

"In Sanford we've got the old Sanford and the new Sanford," she says.

"The old Sanford has controlled everything that has gone on for the last 100 years. At one point there were two communities here - black and white.

"But now there's a third: Hispanic. And the Hispanic community and the black community have joined forces.

"So the good old boys' powers are weakening and this is what is causing some of the racial undertones."

The town is tense and not looking forward to the trial at all.

Many are more than a little worried that the strains will get greater as the trial goes on. They fear that whichever way the verdict goes, it will make a lot of people very angry.