SANFORD, Fla. -- The issue of race in the deadly shooting of a black U.S. teenager was a prominent focus of questioning as jury selection in George Zimmerman's trial entered its second week.

Civil rights leaders led national protests after Zimmerman's fatal shooting of 17-year-old Trayvon Martin last year and as 44 days passed before the arrest of Zimmerman, who identifies himself as Hispanic.

One juror questioned Monday was a middle-aged white woman who described the protests as unsettling and speculated that there could be further marches if Zimmerman isn't convicted of second-degree murder. The candidate, who said she has a biracial grandson, also said she was unsure whether Zimmerman racially profiled Martin because it was dark and the teen was wearing a hoodie, possibly making it difficult to see his race.

Zimmerman, a neighbourhood watch volunteer, was driving through his gated Florida community when he spotted Martin walking back from a convenience store to a home belonging to his father's fiance. Zimmerman called a nonemergency police number, followed Martin and at some point there was a fight between them that left Martin dead.

Zimmerman is pleading not guilty, claiming self-defence.

Prosecutors and defence attorneys are seeking a pool of 40 potential jurors who have been screened for any influence of pretrial publicity before moving to a second round of questioning.

Separately, the judge must decide whether to allow voice identification experts to testify about screams captured on emergency calls made by neighbours during the confrontation between Martin and Zimmerman. Thus far, experts have reached mixed conclusions about whether the screams belong to the teen or the neighbourhood watch volunteer. Defence attorneys don't want the experts to testify.

Testimony from audio experts is expected to continue through jury selection this week.