Those on the streets cannot accept that a man can shoot dead a boy on his way home and not pay some penalty. Neither do they believe that race was not a factor in Martin's death and Mr Zimmerman's acquittal.

''Only in America can a dead black boy go on trial for his own murder,'' the New York writer Syreeta McFadden tweeted, capturing the protesters' mood.

The debate has already fractured the US along familiar fault lines, with elements of the political left backing the National Association for the Advancement of Coloured People's call for an investigation into possible civil rights violations, and the right accusing them of playing the race card.

The Justice Department says it is restarting its investigation into the 2012 shooting of Martin to consider possible separate hate crime charges against Mr Zimmerman, The New York Times has reported.

Tension is high and bleak memories of race riots that have plagued this country have been reawakened. In this context it is perhaps understandable that another crucial debate is being drowned out.

People appear to have forgotten that when race first became a factor in this case it was because for six weeks after the shooting Mr Zimmerman was neither arrested nor charged.

In large part this was due to Florida's so-called stand-your-ground law, introduced in 2005, that made shooting people dead on that state's streets far more likely to be legal than it used to be.

As is so often the case, Florida was an early adopter of a law being championed by conservative activists and, in this case, drafted by the National Rifle Association. Stand-your-ground laws, known by critics as shoot first laws, extend the so-called castle doctrine from the home to the street, making it legal to use lethal force against someone in public if you ''reasonably'' believe yourself to be at significant risk.

In doing so they reverse the onus of proof. Where once someone who killed a person had to prove they had acted in self-defence, under stand-your-ground laws the prosecution must prove the accused did not feel they were at risk.

The laws also expressly remove the requirement that once existed in the use of self-defence that a person retreat if given the opportunity. That is, under stand-your-ground laws, killing someone in a fight need not be the last option.

Also, given that the sole witness to such incidents are often the accused and the person they have killed, stand-your-ground is proving quite a handy defence.

Defence lawyer and former prosecutor Matt Mangino says that under stand-your-ground laws it could well be legal to get into a dispute with a neighbour, allow the quarrel to descend into a scuffle and, if you feared significant harm, to shoot him dead.

In a case that is to be tried soon in the Florida town of Port St Joe, Walt Butler, a white man, is using stand-your-ground in his defence for shooting his neighbour Everett Gant, a black man who had confronted Mr Butler about a racial slur against a child in their apartment complex.

Mr Butler shot Mr Gant between the eyes with a rifle, called police, then finished making his dinner, a local newspaper reported.

Mr Maningo said he has come across cases in which drug dealers use stand-your-ground as a defence after shootings.

Since Florida introduced its law, 30 other states - almost all of them conservative - have followed suit. Many simply copied Florida's legislation or used draft laws written by the American Legislative Exchange Council, a conservative group that seeds laws among the states.

In the end, Mr Zimmerman did not resort to the stand-your-ground argument in court, instead relying on straightforward self-defence. Nonetheless, the ease with which self-defence is invoked in murder cases is clearly on Floridians' minds.

Less than half an hour after the verdict on Mr Zimmerman was handed down, Florida's Senate's Democratic leader Chris Smith issued a statement calling for a re-examination of all self-defence laws.

''The fact that a child is dead and an armed man can now walk free without so much as a backward glance sends the wrong message to Florida and its citizens,'' he said.

''If someone makes the claim of self defence and the only other witness to the confrontation is dead, there needs to be a higher standard for proving that the use of deadly force was justified.''

There is little chance of any real debate in Florida, where the state legislature remains in the hands of the conservative Tea Party.

Columbia University law professor Jeffrey Fagan says stand-your-ground laws are perfectly suited to a conservative world view. They speak of mistrust of government and its agencies; they privatise public space and promote individualism at its most rugged, he says.

He notes that they are an extension of a long history of American citizens using violence as self-help.

''Lynchings, riots, civil disobedience and vigilantism are all expressions of individual or collective action that reject both legal norms and the authority of state actors,'' he has written.

Murder rates have not risen in Florida since stand-your-ground laws were introduced, and supporters see this as evidence that they are working safely and as intended.

One man who has studied them more broadly, Texas A&M University economist Mark Hoekstra, disagrees. ''These laws lower the cost of using lethal force,'' he told National Public Radio in January. ''Our study finds that homicides go up by 7 to 9 per cent in states that pass the laws, relative to states that didn't pass the laws, over the same time period.''

Although Mr Zimmerman's case hinged on self-defence, stand-your-ground language still found its way into the court case, with instructions given to the jury declaring, ''If George Zimmerman was not engaged in an unlawful activity and was attacked in any place where he had a right to be, he had no duty to retreat and had the right to stand his ground and meet force with force, including deadly force if he reasonably believed that it was necessary to do so to prevent death or great bodily harm to himself or another, or to prevent the commission of a forcible felony.''

As long as force meets deadly force, and as long as citizens are relieved of the ''duty to retreat'', Professor Fagan expects more people to die as Trayvon Martin did.

Meanwhile, it has been confirmed that Mr Zimmerman is eligible to have his handgun returned to him.