(CNN) - A key piece of evidence was at the center of George Zimmerman's murder trial Friday.
Trayvon Martin's mother and brother listened in court to a 911 call on the night of the shooting and said they recognized the screaming on the recording as coming from the slain teen.
Zimmerman is charged with second-degree murder. His defense team argues that he shot the teen in self-defense.
Lawyers on both sides want to convince the jury of who was doing the screaming, Zimmerman or Martin.
The audio was played in court for Martin's mother, Sybrina Fulton. She said she recognized the screaming as that of "Trayvon Benjamin Martin."
Defense attorney Mark O'Mara asked Fulton if she had no doubt that it was her son screaming.
"Absolutely," she said.
Martin's older brother, Jahvaris Fulton, 22, also took the stand Friday.
Prosecutor Bernie de la Rionda asked him if he recognized the voice on the tape.
"My brother's," Fulton said, adding that he had "heard him yell" before, but "not like that."
The question of whether Zimmerman's actions were self-defense or murder could rest on who was the person screaming.
As the prosecution drew closer to the conclusion of its arguments, de la Rionda called to the stand the man who performed the final autopsy on Martin.
Shiping Bao led the court through a series of photos of Martin's body, pointing out his injuries and explaining what he could conclude from his examination.
Bao's conclusion is that Martin was shot at an intermediate range and that the bullet went right through his heart.
"There is no chance he could survive. Zero," Bao said.
It is possible that Martin was immobile, but remained alive for up to 10 minutes after being shot, Bao said. He would have suffered and felt pain during that time, he said.
Martin, 17, was fatally shot on February 26, 2012.
The shooting put a national spotlight on Zimmerman's hometown of Sanford and sparked fresh debates about race relations and gun laws in America.
Zimmerman is Hispanic; Martin was African-American.
An initial decision not to pursue charges against Zimmerman led to the dismissal of the town's police chief and the appointment of a special prosecutor, who accused the neighborhood watch volunteer of unjustly profiling and killing Martin.
Zimmerman now faces a second-degree murder charge in Martin's death. He has pleaded not guilty and is free on $1 million bond.