Trayvon Martin's father testified Monday that he never denied it was his son's voice screaming for help on a 911 call, contradicting police officers' earlier testimony at George Zimmerman's second-degree murder trial.

Tracy Martin was the latest in a series of witnesses called by lawyers on both sides as they seek to convince jurors of who was the aggressor in the nighttime confrontation that left Trayvon Martin, an unarmed black teenager, dead in February 2012. Later in the day, the Florida judge ruled that defense 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 a 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 Sanford, Fla. police investigator who probed 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-defense claim. Relatives of Martin's and George Zimmerman's have offered conflicting opinions about who is heard screaming.

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 defense attorneys said it was relevant because Zimmerman believed Martin was under the influence at the time he spotted him in his neighborhood. 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 defense attorneys from presenting an animated depiction of the fatal fight between Martin and Zimmerman. Prosecutors had requested that the animation commissioned by the defense not be mentioned or played at Zimmerman's trial, claiming it would only confuse jurors. The judge hadn't made a ruling Monday afternoon.

Most of the day was taken up with testimony by a series of Zimmerman's friends, called to testify that the screams on the recording were his. The recording of 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.

The emergency call captured the confrontation between Zimmerman and Martin shortly before Zimmerman fatally shot the 17-year-old Martin. 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 neighborhood watch volunteer, has pleaded not guilty to second-degree murder and says he shot Martin in self-defense during an altercation in the gated townhome complex in Sanford where he lived. Martin was there visiting his father and his father's fiancee.

Prosecutors contend that Zimmerman, who identifies himself as Hispanic, was profiling Martin and perceived the teen as someone suspicious in the neighborhood, which had been the site of a series of break-ins. The case sparked protests because police did not charge Zimmerman for 44 days and it touched off a nationwide debate about race and self-defense.

After the call was played for Zimmerman's friend Sondra Osterman, defense 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 best friends of Zimmerman and his wife.

Under cross-examination, prosecutor Bernie de la Rionda implied that Osterman and her husband, Mark, had a stake in the outcome of the trial because they had written a book about Zimmerman's case and were donating the proceeds to their friend.

Mark Osterman took the witness stand after his wife and said it was Zimmerman's voice screaming when the 911 call was played for him in the courtroom.

Former co-worker Geri Russo also testified it was Zimmerman yelling on the call, as did John Donnelly and Leanne Benjamin, a married couple who became good friends with Zimmerman and his wife.

The prosecutor also played for Sondra Osterman a nonemergency police call that Zimmerman made to report Martin walking through his neighborhood. In the call, Zimmerman uses the words, "F------ punks. These a-------. They always get away." Sondra Osterman identified the voice as Zimmerman's.

When asked by O'Mara if she detected ill will, spite or hatred in his voice, she answered no.

Prosecutors must show that Zimmerman acted with ill will, spite or a depraved mind in order to get a second-degree murder conviction.

Defense attorneys also called the owner of a gym where Zimmerman had gone to lose weight to explain to jurors the mixed-martial arts fighting method called "ground and pound." Defense attorneys have said that Martin slammed Zimmerman's head into the sidewalk while he was on top of him in "a ground and pound" maneuver. To demonstrate the move, gym owner Adam Pollock straddled O'Mara on the courtroom floor. Pollock testified that Zimmerman trained in the form of fighting known as grappling but was an unaccomplished fighter.

"He was physically soft," said Pollock when asked to rank Zimmerman's athletic skill on a scale of 1 to 10.

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Follow Mike Schneider on Twitter at http://twitter.com/MikeSchneiderAP .