SANFORD, Fla. -- A video security expert has taken the witness stand in the George Zimmerman murder trial.
Greg McKinney is the first witness Friday as the trial enters a fifth day of testimony.
McKinney's company monitored the security cameras at the townhouse complex where Zimmerman fatally shot 17-year-old Trayvon Martin last year.
Zimmerman is pleading not guilty to second-degree murder, claiming self-defense.
Jurors already have been exposed to some of the state's biggest pieces of evidence, including the 911 call featuring cries for help prosecutors believe came from Martin.
On Thursday, a friend of Martin who had been on the phone with him when he was shot testified about what she heard during his confrontation with Zimmerman.