SANFORD, Fla. -

Tracy Martin, the father of the Miami teen who was shot and killed by George Zimmerman in Sanford, was called to testify on Monday.

[Chat recap:  Tony Pipitone in courtroom | SPECIAL SECTION:  Zimmerman trial]

Trayvon Martin's father was questioned by defense attorney Mark O'Mara about how he said the screaming in the background of a 911 call was not his son.

Tracy Martin said he recalls lead investigator Chris Serino asking him "do you recognize the voice?" Tracy Martin says he answered "I can't tell" and never said "no it wasn't my son's voice."

Tracy Martin denied saying he listened to a "cleaned up" or "enhanced" version of the tape. He also said he never told Martin's mother, Sybrina Fulton, about listening to the tape.

"After listening to it maybe 20 times, I said that I knew it was Trayvon's voice," Tracy Martin said of the March 16, 2012 tape playing in the Sanford mayor's office.

During cross-examination, Tracy Martin called Trayvon Martin, "my best friend in life."

"It's still hard to believe this day that he's dead," Martin said.

The defense objected to the state broadening its questions beyond the scope of direct. The objection was apparently overruled at bench as the state asked about identifying his son's body in a photo the day before the Sanford Police Department meeting.

"My world has been turned upside down," Tracy Martin said. "I was listening to my son's last cry for help ... his life being taken."

Tracy Martin said he played the screams repeatedly, "trying to figure out ... why did the defendant get out of his vehicle and chase my son."

The defense then called former Sanford Police Chief Bill  Lee to testify. Lee said it's proper to do lineups with individual witnesses, not as a group, "so their decision is not influenced" by others.

Lee said he was intentionally excluded from playing tape for Martin family by city manager and that he offered to be present.

"Evidence and testimony gave us an indication" of whose voice was screaming" but lee said they were looking for corroboration.

After Lee's testimony, Nelson dismissed the jury for the day to conduct a Richardson hearing for a state motion claiming they weren't told one of the defense witness' testimony had changed.

Earlier on Monday, O'Mara questioned Serino on when Tracy Martin, arrived at the Sanford Police Department. Serino said two days after the shooting he played the 911 audio at his desk. Serino described Tracy Martin as giving an emotional response and when he asked if Tracy Martin recognized the screams, Martin looked away and said under his breath, "no."

During cross-examination, prosecutor Bernie de la Rionda asked Serino if he had warned Tracy Martin he would hear the gunshot in the background of the 911 call.

Serino said he didn't recall warning Tracy Martin and played all 911 calls, including the emotional 911 call from witness Jane Surdyka. Serino said he expected Tracy Martin to be very emotional.

Serino said he asked Tracy Martin either "do you you recognize the voice?" or "is that your son's voice in the background?" De la Rionda asked if Tracy Martin's response about the 911 calls "could be construed as denial. Serino agreed.

Tracy Martin has previously said call quality and environment at SPD prevented him from being certain it was his son's voice, but later said it was.

During redirect, Serino told O'Mara Tracy Martin's denial "became significant in the investigation" because other witnesses had information it was Zimmerman.  Serino said Zimmerman didn't deny it was his voice, in context, he was saying his voice sounded unusual to him and not clearly recognizable.

Sanford Police Department Detective Doris Singleton testified that Martin's father, Tracy Martin, was at the department asking why no one was arrested. The state objected to O'Mara asking Singleton what Tracy Martin said as hearsay.

Singleton was called again after Serino's testimony to back up fellow officer Serino's recollection that Tracy Martin didn't recognize the vioce.

Singleton testified that Tracy Martin was "very upset, very sad, he hung his head and cried." Singleton said Tracy Martin listened to the 911 call with screams at the police station two days after the shooting.

Jurors then heard from Adam Pollock, the owner of the gym where Zimmerman took mixed martial arts classes. Pollock testified about the "ground and pound" and demonstrated the MMA move on O'Mara.