SANFORD, Fla. -

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 said that he merely told officers he couldn't tell if it was his son after his first time listening to the call.

"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.

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Tracy Martin's testimony came after 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 whose voice is on the tape is important to both sides because it would help jurors decide who was the aggressor in the confrontation that left Martin dead. Relatives of Martin's and George Zimmerman's have offered conflicting opinions about who is heard screaming.

During cross examination of Serino, prosecutor Bernie de la Rionda suggested that Tracy Martin may have been in denial about his son's death and uttered, 'no."

"It could be perceived as denial," Serino said.

The investigator's testimony was just the latest effort to determine who was crying for help on the 911 calls. A series of Zimmerman's friends on Monday testified that the screams on the recording were their friend, and the 911 call was played multiple times in the courtroom.

After the call was played for 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 the best friends of Zimmerman and his wife.

The emergency call captured the confrontation between Zimmerman and Martin shortly before Zimmerman fatally shot the teen. 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.

Prosecutors had wanted to introduce as witnesses two audio experts who said the voice belonged to Martin and ruled out Zimmerman's voice. But Judge Debra Nelson prohibited the audio experts from testifying, saying their methods were unreliable.

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

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

Prosecutors were also seeking Monday to stop defense attorneys from presenting an animated depiction of the fatal fight. Their motion requests that the animation not be mentioned or played at Zimmerman's trial, claiming it would only confuse jurors. They said the animation doesn't show a murder weapon, only approximates positions based on witness accounts and artificially depicts lighting conditions.

Defense attorneys hadn't immediately filed a response. Prosecutors said in their motion that the animation commissioned by the defense was created by employees of the animator re-enacting the fight wearing motion-capture suits.

Under cross-examination, prosecutor Bernie de la Rionda implied that Sondra 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 to testify about how Zimmerman had chosen and purchased his firearm. He testified that Zimmerman could shoot with both hands, and he also said he recommended keeping the gun loaded.

He 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 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.