Tracy Martin, the father of Trayvon Martin, testifies in George Zimmerman's trial at the Seminole Circuit Court, in Sanford, Fla., Monday, July 8, 2013. Zimmerman is charged with second-degree murder in the fatal shooting of his son, in 2012. (AP Photo/Orlando Sentinel, Joe Burbank, Pool)

SANFORD, Fla. - Swaying jurors about whose screams are on a 911 call that captured the fatal struggle between George Zimmerman and Trayvon Martin has become the primary goal of prosecutors and defence attorneys in Zimmerman's second-degree murder trial.

Defence attorneys called five of Zimmerman's friends Monday to testify that it was the neighbourhood watch volunteer's voice yelling for help on the call, and they played the recording five times for jurors. Zimmerman's attorneys also called two police investigators who contend Martin's father, after hearing a recording of the call, initially said it wasn't his son. Zimmerman's attorneys then put Martin's father, Tracy Martin, on the witness stand, and he denied ever saying the screams for help weren't his son.

Later in the day, the judge ruled that defence attorneys may present evidence to the jury that Trayvon Martin had marijuana in his system when he died.

The teen's father testified that he merely told officers he couldn't tell if it was his son after his first time listening to the call, which captured the audio of fight between Martin and Zimmerman.

"I never said that wasn't my son's voice," said Tracy Martin, who added that he concluded it was his son after listening to the call as many as 20 times.

Before Tracy Martin took the witness stand, the lead investigator probing Martin's death testified that the father had answered "no" when the detective asked if the screams belonged to Trayvon Martin. Officer Chris Serino played the 911 call for Tracy Martin in the days immediately following Trayvon Martin's death in February 2012.

"He looked away and under his breath he said 'no'," Serino said of Tracy Martin.

Officer Doris Singleton backed up Serino's account.

Convincing the jury of who was screaming for help on the tape is important to both sides because it would help jurors evaluate Zimmerman's self-defence claim. Relatives of Martin's and Zimmerman's have offered conflicting opinions about who is heard screaming. Zimmerman's mother and uncle testified last Friday it was Zimmerman screaming. Martin's mother and brother also took the witness stand last Friday to say the voice belongs to Martin.

Zimmerman himself once said during a police interview that the screams didn't sound like him, though he and his family later said the screams were his.

Zimmerman, a former neighbourhood watch volunteer, has pleaded not guilty to second-degree murder and says he shot Martin in self-defence during a scuffle in the townhome complex where he lived. Martin was there visiting his father and his father's fiancee.

Prosecutors contend Zimmerman was profiling Martin and perceived the teen as someone suspicious in the neighbourhood, which had been the site of a series of break-ins.

Most of the day was taken up by a series of Zimmerman's friends called to testify that the screams on the recording were his, and the 911 call was played multiple times in the courtroom. A gym owner who trained Zimmerman also described him as physically soft and an inferior fighter.

After the call was played for Zimmerman's friend Sondra Osterman, defence attorney Mark O'Mara asked who it was.

"Yes, definitely. It's Georgie," said Osterman, who testified she first met Zimmerman in 2006 while working with him at a mortgage company. Osterman and her husband, Mark, describe themselves as the best friends of Zimmerman and his wife.

Late in the day, Judge Debra Nelson made a key ruling out of the presence of the jury.

The judge denied a prosecution request to keep out parts of a toxicology report that shows Trayvon Martin had small amounts of marijuana in his system. Prosecutors argued the information would be prejudicial. But defence attorneys said it was relevant since Zimmerman believed Martin was under the influence at the time he spotted him in his neighbourhood. Nelson had ruled before the trial that mention of marijuana wouldn't be allowed in opening statements.

Next, the judge considered a prosecution motion to stop defence attorneys from presenting an animated depiction of the fatal fight between Martin and Zimmerman. Prosecutors' motion requests that the animation commissioned by the defence not be mentioned or played at Zimmerman's trial, claiming it would only confuse jurors. The judge postponed making a ruling until after prosecutors could take a deposition of the defence witness who made the animation.

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News from © The Associated Press, 2013