SANFORD, Fla. -

In the second row, near the panel of jurors Tracy Martin has cried in silence many times in the courtroom during the George Zimmerman trial. 

Pastors, attorneys, reporters surround him. Sometimes he discreetly puts his head down and his hand over his face.  On Monday, jurors finally heard from him. His eyes were red, but despite his aura of sadness, he did not cry on the stand.

Defense attorneys asked him why it took him so long to identify his 17-year-old son Trayvon Martin as the person recorded in a 911 call screaming for help.

"I never said that's not my son's voice," Martin testified. He said his response was, "I can't tell."

Two Sanford police officers -- Doris Singleton and Chris Serino --  testified they saw and heard the distraught father say that was not his son. 

Martin said he wasn't aware Singleton was there and Serino misunderstood his shock for conviction. But that after listening to the recording many times, he was sure it was his son.

Defense attorneys called several of Zimmerman's friends and family members to testify that they recognized the screams as those of the crime watch volunteer.

Martin also said the day his son died his world "turned upside down" and could not understand why Zimmerman got off his car and chased his son.  The day he listened to the recording for the first time, he said he was in the police station voluntarily.

"I was listening to my son's last cry for help. I was  listening to his life being taken," Martin said.  "And I was coming to come to grips that Trayvon was here no more. It was just tough."