George Zimmerman would surrender to authorities if he is charged in the Florida shooting death of unarmed black teenager Trayvon Martin, his attorney said on Monday, adding he was preparing for trial in the case that has captivated the U.S. public.

If he's charged, he will be arrested and he will turn himself in, attorney Craig Sonner told Reuters in a telephone interview. However it goes, he's not hiding from the authorities. If he is asked, he will turn himself in. There's not going to be a manhunt or anything like that.

Zimmerman, a neighbourhood watch volunteer, fatally shot Martin on the night of February 26, saying he had acted in self-defence. Police released him without charge and said there was no evidence to contradict Zimmerman's story that he was attacked by Martin, 17.

The case has riveted the country, prompting demonstrations in numerous cities demanding Zimmerman's arrest. Critics believe Zimmerman, who is half white and half Hispanic, was motivated by racial profiling in deciding to follow Martin and that race played a role in police accepting his version of events.

Sonner and Zimmerman's relatives say he is not racist and has been unfairly vilified. They said he feared for his life during his altercation with Martin and was justified in using deadly force.

Zimmerman remains free but in hiding. They can contact me at any time if authorities need to locate him, Sonner said.

Special prosecutor Angela Corey has taken over the case and could announce soon whether she intends to charge him with a crime.

We're preparing for trial, Sonner said. We've done all we can except ask for discovery (evidence-sharing) from the state, which is not available to us yet because Zimmerman has not been charged.

INVESTIGATION UNDER WAY

Following intense public criticism and a vote of no confidence by the Sanford City Commission, Police Chief Bill Lee resigned temporarily on March 22. He said the uproar surrounding him had become a distraction to the investigation.

That same day, State Attorney Norm Wolfinger also said he was taking himself off the case with the intent of toning down the rhetoric. Wolfinger on Monday broke a long silence about the case, denying reports he quashed police intentions to charge Zimmerman with manslaughter.

The special prosecutor's team took over, retracing many of the steps previously taken by police.

Sanford police and Corey's office have declined all comment in recent days.

Neither Sonner nor Benjamin Crump, a lawyer representing Martin's family, would speculate on when the investigation might conclude.

Reuters has observed Corey's investigators in several places, including the neighbourhood of the shooting, where they interviewed witnesses; the Sanford police station; and a hotel where they interviewed Trayvon Martin's parents, Tracy Martin and Sybrina Fulton.

Trayvon Martin was returning to his father's fiancée's home at a gated community on February 26 when Zimmerman saw him walking in the rain wearing a hooded sweatshirt and considered him suspicious. Against the advice of a police operator, Zimmerman followed Martin and a fight occurred. Witnesses heard screams for help and then a gunshot, sounds captured on 911 emergency calls by neighbours.

Zimmerman told police that Martin approached him, punched him in the nose and slammed his head against a concrete walkway before he fired in self-defence.

Forensic experts say the voice heard on the tape calling for help was not Zimmerman's, according to a report in Sunday's Orlando Sentinel. The newspaper asked voice recognition experts to examine the screams and compare them to Zimmerman's voice on calls he made to 911 before the shooting. The results did not match.

No tests were done comparing the tape with Trayvon Martin's voice because the newspaper could not obtain a sample of the teen's voice.

ABC News on Monday reported a newly enhanced version of surveillance video from the night of the shooting in the Sanford police station more clearly shows what appears to be an injury on the back of Zimmerman's head.

When the video first appeared last week, no injury was readily visible, leading critics of Zimmerman to accuse him of lying about having the back of his head slammed against the concrete repeatedly.

The enhancement was conducted by Forensic Protection Inc., a California company that enhances video and audio.

(Reporting by Daniel Trotta; Editing by David Adams and Bill Trott; Desking by Lisa Shumaker)