A Florida special prosecutor was expected to announce a criminal charge on Wednesday against the neighbourhood watch volunteer who shot and killed an unarmed black teenager in a racially charged case that has captivated the United States.

Special prosecutor Angela Corey called a news conference for 6 p.m. ET (11:00 p.m British Time).

NBC News reported Zimmeran was in custody.

The decision whether to charge shooter George Zimmerman, 28, in the death of Trayvon Martin, 17, has rested with Corey, the special prosecutor appointed by Florida's governor to investigate a homicide that has prompted civil rights demonstrations around the country.

Zimmerman said he acted in self-defence during a confrontation in a gated community in the central Florida city of Sanford on February 26. Police declined to arrest Zimmerman, citing Florida's Stand Your Ground law, which allows people to use deadly force when they believe they are in danger of getting killed or suffering great bodily harm.

The shooting that took place 45 days ago received only scant local media attention at first and went unnoticed nationally until Martin's parents and lawyers kept making public calls for Zimmerman's arrest, eventually leading to a firestorm of media coverage, and celebrity tweets, and a comment from President Barack Obama: If I had a son, he would look like Trayvon.

The disputed facts of the case have been picked apart endlessly by television commentators while dominating the headlines and reigniting a national discussion about guns, self-defence laws and what it means to be black in America.

Zimmerman went into hiding shortly after the shooting. His former lawyers said he left Florida but has remained in the United States.

Zimmerman's relatives and supporters 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.

The case took an unexpected turn on Tuesday when the two lawyers representing Zimmerman, Craig Sonner and Hal Uhrig, dropped him as a client because they had lost contact with him, saying Zimmerman had contacted Corey's office directly in an apparent attempt to resolve the case on his own.

They also expressed concern for his mental and physical health.

I still believe he is a good person, Sonner told Reuters on Wednesday. I believe he's not a racist. I believe he acted in self-defence.

(Additional reporting by Barbara Liston; Writing by Daniel Trotta; Edited by Prudence Crowther)