Friends testify it's Zimmerman on 911 call for help SANFORD, Florida (Reuters) - The dispute over who screamed for help the night George Zimmerman killed Trayvon Martin dominated Zimmerman's trial on Monday, with five friends saying the voice belonged to the neighborhood watch coordinator and the victim's father insisting it was his son's dying plea. The testimony of Zimmerman's friends, at Florida's Seminole County Court, lent support to his claim he killed the unarmed black teenager in self-defense. His second-degree murder trial could turn on the identity of the voice heard in the background of a 911 emergency call on the night of February 26, 2012, to police in Sanford, Florida, where Zimmerman, 29, shot Martin, 17, triggering a national debate on race and guns in America. Martin's mother and brother previously told the jury of six women they recognized the voice of Martin, when the prosecution concluded its side of the case on Friday. They were followed by Zimmerman's mother and uncle, who testified for the defense it was their relative pleading for help. That dispute continued in court on Monday, when the five Zimmerman friends said they recognized the screams as coming from him. The defense team then called two police detectives who had played the audio for Martin's father, Tracy Martin, in the days after the shooting. The detectives testified they asked Tracy Martin if he could recognize the voice and that he said, "No." But Tracy Martin contested that account on Monday, saying, "I never said 'No, that wasn't my son's voice.'" When he had the chance to listen to the recording repeatedly, he said, he believed it was his son. "I was listening to my son's last cry for help; I was listening to his life being taken," Martin said. Tracy Martin's testimony also gave him the opportunity to grieve about his son's loss before the jury, saying he lost his "best friend," and to ask the question, "Why did the defendant get out of his vehicle and chase my son?" As the defense case neared its end, it remained uncertain whether Zimmerman would choose to testify. Zimmerman faces up to life in prison if convicted of second-degree murder, although either side can request that the jury also consider the lesser offense of manslaughter, with a maximum penalty of 30 years. In a defeat for the prosecution, Judge Debra Nelson ruled that defense lawyers can introduce evidence that Martin had the active ingredient of marijuana in his system when he was killed. Toxicology tests showed a THC in Martin's system, and Zimmerman told a police operator just before the shooting that Martin "looks like he's on drugs." The prosecution argued the evidence was prejudicial, and the defense countered that it was relevant given Zimmerman's observation that Martin could be on drugs. Medical Examiner Shiping Bao initially reported the THC level was too slight to affect Martin, but Bao testified outside the jury's hearing last week that his further research in preparation for the trial indicated the drug might have had a slight but unknown effect. Police in Sanford, Florida, at first decided against arresting Zimmerman, who is white and Hispanic, accepting his claim of self-defense. It also drew attention to Florida's "Stand Your Ground" self-defense law, which police cited in initially declining to arrest Zimmerman, who was licensed to carry a concealed weapon that was fully loaded with hollow-point rounds. The screams were recorded in the background of a 911 call placed by a neighbor who called to report two men fighting. The screams end when a shot rings out from Zimmerman's Kel Tec 9mm pistol. An FBI voice recognition expert testified last week the screams were too short and the audio of too poor quality to apply standard scientific voice identification techniques, saying the next best method to identify the voice was by a listener who knew the person his whole life. Defense lawyers followed up on Monday by calling the five Zimmerman friends, who said it was Zimmerman's voice. Among them was John Donnelly, who said Zimmerman was "like a son" and who donated $3,000 to Zimmerman's defense and bought him $1,700 worth of clothes for his trial. As a medic in the Vietnam War in the 1960s, Donnelly said, he developed an ability to recognize which of his fellow soldiers was screaming for help. "And I wish to God I didn't have that ability to understand that," Donnelly said. The defense also called a gym owner, Adam Pollock, who told the jury Zimmerman was "soft" and lacked proficiency in fighting after taking classes in grappling and boxing for a year. Although Zimmerman lost 50 to 80 pounds (22 to 36 kg), he lacked the muscle or fighting skill to train against an opponent in the ring, he said. The testimony could be helpful in explaining how Zimmerman, at around 200 pounds (90 kg), could have lost a fight to Martin, who weighed 158 pounds (71 kg) at his autopsy. Since his arrest, Zimmerman has ballooned to more than 300 pounds (136 kg) due to what his lawyer called the stress of going on trial. Writing by Daniel Trotta; Editing by Dina Kyriakidou and Steve Orlofsky