Tracy Martin took the stand on Monday at the trial of George Zimmerman and denied that he told police officers the screams for help heard on a 911 call weren’t those of his son Trayvon.

On the 20th day of testimony, Defense attorneys called several witnesses—including Martin—to try to convince the jurors that it was Zimmerman. Who was screaming for help on the tape is important to both sides because it would help jurors evaluate Zimmerman’s self-defense claim. Last Friday, Sybrina Fulton, Trayvon’s mother, and his brother, Jahvaris, testified that it was the teen’s voice; Zimmerman’s mother and uncle testified that it was Zimmerman.

The call captured the fight between Trayvon and Zimmerman. Zimmerman has pled not guilty to second-degree murder, claiming self-defense.

Martin said that two days after the shooting, he went to the Sanford Police Department to get an update on the case and investigator Chris Serino played him the 911 tape—which contained the sound of the fatal gunshot. Officer Serino then asked Martin if the screams were Trayvon’s.

“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.’”

He told the court how difficult it was listening to the call. “I never said that ‘No, that wasn’t my son’s voice.’ I was listening to my son’s last cry for help. I was listening to his life being taken.’”

Martin said he listened to the 911 call 20 times. Trayvon “was my best friend in life. And to have him gone is tragic,” he said. “Why did the defendant get out of his vehicle and chase my son?”

Earlier, Officer Chris Serino, the lead investigator on the case, had testified that Martin told them after listening to the 911 call that the person screaming for help wasn’t Trayvon. He said that after listening to the call, Martin “looked away and under his breath he said ‘no’.”

Police officer, Doris Singleton, whose cubicle was near Serino’s, testified that she overheard the conversation and backed up Serino’s account.

A parade of witnesses—all friends of Zimmerman—said the voice they heard screaming on the 911 call was Zimmerman’s.

Zimmerman’s best friend Mark Osterman, an air marshal, said he gave Zimmerman advice on buying a gun and had told him the Kel-tec 9mm—the one he used to killed Martin—was a good choice for self-defense. Following the shooting, the Ostermans hid Zimmerman in their home for six weeks.

Last year, Osterman and his wife Sondra, who also testified, published a book about their friendship with Zimmerman, Defending Our Friend: The Most Hated Man in America, and said they plan to give money earned from book sales to Zimmerman after the trial.

Another witness, John Donnelly, said he donated $3,000 to Zimmerman’s defense fund, and spent $1,700 on three suits, shirts and ties for him to wear to court.

The former police chief of Sanford, Bill Lee Jr., who lost his job in the wake of the shooting, also testified about the 911 tape.

Also on Monday, Judge Debra Nelson ruled that toxicology reports that show the amount of THC—the active ingredient in marijuana— in Trayvon’s blood when he was killed can be admitted, along with expert testimony. No one knows when Martin might have smoked marijuana. Experts also disagree on what the levels mean what effect it might have had on him.

Expert testimony on the toxicology report is scheduled to start today.