Joe Burbank / Pool via AP The parents of Trayvon Martin, Tracy Martin, right, and Sybrina Fulton, listen to the testimony of Sanford police officer Chris Serino during the George Zimmerman trial in Seminole Circuit Court in Sanford, Fla., on Monday.

Trayvon Martin’s father on Monday denied that he told police the person screaming in the background of a 911 call was not his son – contradicting the testimony of two officers who took the stand in the George Zimmerman trial.

Tracy Martin was called to the stand by the defense, which is hoping to convince jurors that Zimmerman is the one who was yelling for help, bolstering its assertion that he fired his gun because he was being attacked by the teen.

The dad said that two days after the fatal confrontation, he went to the Sanford, Fla., police department for an update and investigator Chris Serino played him the tape – which contained the crack of the fatal gunshot.

“As best as I recall, after he played the tape, he basically just said, ‘Do you recognize the voice?’” Martin said.

“My response was… I didn’t tell him, ‘No, that wasn’t Trayvon,’” the father added.

“The chairs had wheels on them and I kind of pushed away from the table and just kind of shook my head and said, ‘I can’t tell.’

“I never said that ‘No that wasn’t my son’s voice.’”

He said he heard the tape again weeks later in the Sanford, Fla., mayor’s office and after replaying it perhaps 20 times, “I said that I knew that it was Trayvon’s voice.”

Earlier, Serino and a second officer testified that Tracy Martin told them the person yelling for help was not his son.

“There is no doubt he was telling us that -- that didn’t sound like his son to him,” Officer Doris Singleton told the court.

Serino said that when he asked if the voice was his son’s, the father’s response was “verbal and non-verbal.”

“He looked away and under his breath, as I interpreted it, said, ‘No.’”

The voice identification is important because both sides are expected to argue that the person pleading for help is the one being assaulted during the Feb. 26, 2012, clash between Zimmerman, 29, and Martin, 17.

Zimmerman has pleaded not guilty to second-degree murder and says that he shot Martin after being attacked. Prosecutors contend Zimmerman profiled and followed the unarmed Martin before shooting him dead.

Joe Burbank-Pool/Getty Images LeAnne Benjamin, a friend of George Zimmerman, smiles while identifying him in court July 8.

The judge has ruled that audio experts cannot testify during the trial about who they think is screaming on the call because the voice-recognition technology they used is not reliable enough -- but that people who personally knew Zimmerman or Martin could weigh in.

The defense has put Zimmerman’s mother and uncle and five friends of the former neighborhood watch volunteer on the stand to testify that they recognize the voice.

Prosecutors last week had called Trayvon Martin’s mother and brother, who said they were sure the teen was yelling, but the state did not call Tracy Martin.

On cross-examination, prosecutor Bernie de la Rionda hammered home the idea that the father was distraught after listening to the tape.

“He was hearing for the first time — right — the death of his son?” de la Rionda asked Serino.

Neither Serino nor Singleton wavered from the idea that the father did not recognize the screaming as his son’s.

“He was telling Chris it was not his son’s voice,” Singleton said, adding that she choked up herself while watching Tracy Martin listen to the gunshot that killed his son.

“I have children myself,” she said.

“I remember feeling so awful for him that he would have to hear that.”

The father said his emotions were swirling after he heard the tape.

“I was listening to my son’s last cry for help. I was listening to his life being taken. And I was trying to come to grips that Trayvon was here no more,” he said.

Beyond the 911 call, the 10th day of testimony included several other developments:

Judge Debra Nelson ruled that evidence about the amount of THC – the main ingredient in marijuana – in Trayvon Martin’s blood when he was killed can be admitted, along with expert testimony. Experts disagree on what the levels mean, with respect to when Martin might have last smoked marijuana and what effect it might have had on him when he encountered Zimmerman.

Friend Mark Osterman, an air marshal, said he had given Zimmerman advice on buying a gun and had told him the Keltec 9mm – the one used to killed Martin – was a good choice for self-defense and that he should keep one bullet in the chamber.

Adam Pollock, who owned the gym where Zimmerman took classes, said he instructed him in grappling, the goal of which is to “make the guy say, ‘Uncle.’” He said Zimmerman was unathletic and “soft” when he started and lost between 50 and 80 pounds but never got good enough to box in the ring.

Editor's note: George Zimmerman has sued NBC Universal for defamation. The company strongly denies the allegation.

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