(CBS)--Taking the stand Monday, the father of slain Florida teen Trayvon Martin denied telling police officers that he didn't believe it was his son's voice heard screaming for help on a 911 call that is critical to the murder case against George Zimmerman.
Earlier Monday, investigators Chris Serino and Doris Singleton testified that in their meeting with Tracy Martin two days after his son Trayvon was fatally shot by Zimmerman, the father had said the screaming voice in the background of the 911 call was not his son.
The much-contested call was placed by a neighbor the night of the fatal altercation between Zimmerman and Trayvon Martin, and has been at issue throughout Zimmerman's second-degree murder trial. Several friends have testified that Zimmerman can be heard screaming in the background of the call, but Martin's family members have said it was the teen.
Zimmerman, a former neighborhood watch captain, is charged in Martin's death. He claims he shot in self-defense.
Serino testified that after playing the tape during the Feb. 28, 2012 meeting, he asked Tracy Martin whether the voice in the background was his son's. "He looked away and under his breath, as I interpreted it, said 'no,'" Serino said.
Taking the stand, however, Tracy Martin gave a different account of the events of the meeting at the Sanford Police Department.
"My response was, I didn't tell him, 'No, that wasn't Trayvon,'" Martin said. "I think the chair had wheels on it, I kinda pushed away from the table and kind of shook my head and said, 'I can't tell.'"
Martin described the emotional experience of listening to the tape, on which cries for help and the fatal gunshot can be heard.
"Basically what I was listening to, I was listening to my son's last cry for help, listening to his life being taken. I was trying to come to grips that Trayvon was here no more. It was just tough."
Martin said he later determined that the voice was his son's after listening to the tape several more times.
The call is a crucial piece of evidence because it could provide clues as to who was the aggressor in the fatal Feb. 26, 2012 confrontation.