Sanford, Florida: The trial of a US neighbourhood watch volunteer accused of murdering a black teenager took a fresh twist on Thursday after jurors were told they could convict him of the lesser charge of manslaughter.

“Wannabe cop” George Zimmerman wrongly profiled Trayvon Martin as a criminal, followed him with a gun and provoked him into a fight that resulted in the shooting death of the unarmed black teenager, a prosecutor said on Thursday.

“A teenager is dead,” Florida state prosecutor Bernie de la Rionda told the jury in closing arguments of the murder trial.

“He is dead through no fault of his own. He is dead because another man made assumptions. Because his assumptions were wrong, Trayvon Benjamin Martin will no longer walk on this Earth.” Defence lawyers were due to present their closing arguments on Friday, followed by prosecution rebuttal, after which the jury would begin deliberating on the case, which has captivated and polarised much of the US public.

Time and again, de la Rionda accused Zimmerman of lying about what happened the rainy night of February 26, 2012, when he spotted Martin inside a gated community in the central Florida town of Sanford.

Zimmerman called police, saying he believed Martin was suspicious and noted that there had been break-ins in the neighbourhood.

But Martin was a guest in the home of his father’s fiancée, who lived inside the gated community, and was returning from a nearby convenience store with a snack in preparation for watching the NBA All-Star game.

De la Rionda tried to undermine Zimmerman’s contention that he was not following Zimmerman but rather looking for a street address to relay to police. The prosecutor sought to reveal inconsistencies in Zimmerman’s statements and repeatedly quoted him speaking in police jargon, such as describing Martin as a suspect.

Pointing at Zimmerman, who was seated at the defence table, de la Rionda asked the jury to use common sense to answer the question: “Do you believe there’s an innocent man sitting over there?” “Who started this?” de la Rionda asked. “Who followed who? Who was minding his own business? And of the two, who was armed?” De la Rionda avoided mentioning race but said Zimmerman “profiled” Martin, suggesting he assumed Martin was a criminal because he was black.

He also paraphrased Martin Luther King in trying to defend witness Rachael Jeantel, a young woman of Haitian and Dominican origin who was on the phone with Martin when the teenager encountered Zimmerman.

Jeantel underscored the racial divide in the case by using what de la Rionda called “colourful language,” such as when she quoted Martin as referring to Zimmerman as a “crazy ass cracker.” She also testified that Martin told Zimmerman to “get off,” suggesting Zimmerman initiated the confrontation.

“I had a dream that a witness was judged not by the colour of her personality but by the content of her testimony,” de la Rionda said.

Earlier on Thursday, Seminole County Judge Debra Nelson gave jurors the option of convicting Zimmerman of the lesser charge of manslaughter.

However, in a victory for Zimmerman, Nelson denied a prosecution request to add a second option of third-degree felony murder based on child abuse. Martin, at age 17, was a minor.

Prosecutors wanted the sequestered, all-female jury to have the option of choosing a lesser offense that still carries a potentially lengthy sentence.

Zimmerman could be sentenced to life in prison for second-degree murder and up to 30 years for manslaughter.

The defence preferred an all-or-nothing choice of second-degree murder, confident it had shown Zimmerman acted in self-defence and concerned the jury might opt for what lead defence lawyer Mark O’Mara described as a “compromise verdict.” Zimmerman’s detractors see him as a racial profiler who considered Martin suspicious because the teenager was black, and blame the defendant for unnecessarily pulling out his Kel Tec nine millimetre pistol, which was fully loaded with hollow-point bullets.

Backers of liberal gun laws have rallied behind Zimmerman and helped fund his defence, seeing him as a persecuted hero who personifies the Second Amendment right to bear arms.