Zimmerman, 29, could get life in prison if convicted of second-degree murder for gunning down Martin as the young man walked from a conveniencee store. Zimmerman followed him in his truck and called a police dispatch number before he and the teen got into a fight.
Zimmerman has claimed self-defense, saying he opened fire after the teenager jumped him and began slamming his head against the concrete sidewalk.
Zimmerman, whose father is white and whose mother is Hispanic, has denied the confrontation with the black teenager had anything to do with race, as Martin's family and its supporters have charged.
A Sanford police sergeant who was the second officer to arrive on the scene also testified Tuesday. Sgt. Tony Raimondo said he tried to seal a bullet wound in Martin's chest with a plastic bag and attempted mouth-to-mouth resuscitation.
Bubbling sounds indicated air was escaping the teen's chest, Raimondo said. Martin was pronounced dead a short time later.
During Raimondo's testimony, prosecutors showed jurors a photo of a dead Martin face-down in the grass, another of Martin's body face up with his eyes slightly open, and a third of the bullet wound. Martin's father, Tracy Martin, walked out of the courtroom during the testimony.
Wendy Dorival, former coordinator of the Sanford Police Department's neighborhood watch program, testified how she had worked with Zimmerman to set up a watch in his neighborhood.
When asked by prosecutor John Guy if neighborhood watch participants should follow or engage with suspicious people, she said no.
"They are the eyes and ears of law enforcement," Dorival said. "They're not supposed to take matters into their own hands."
Similarly, Donald O'Brien, president of Zimmerman's homeowners association, said it was his understanding that neighborhood watch members are supposed to "stay at a safe distance" and "let the police handle it."
But Dorival said she was impressed with Zimmerman's professionalism and dedication to his community.
"He seemed like he really wanted to make changes in his community, to make it better," she said.