(CBS) --A man who described himself as a "close friend" of accused murderer George Zimmerman wiped away tears as he said there was no doubt in his mind that the voice screaming in the background of a disputed 911 is the former neighborhood watch captain.
John Donnelly was the seventh witness to testify for the defense that the voice on the call, placed the evening Zimmerman fatally shot Florida teen Trayvon Martin in a Sanford, Fla. gated community, belonged to Zimmerman. Donnelly's wife Leanne Benjamin, former co-worker Geri Russo, Zimmerman's best friend Mark Osterman and his wife Sondra Osterman, and Zimmerman's mother and uncle have all said the voice belongs to the 29-year-old.
Who is screaming on the call has been a key point of contention because it could provide clues as to who was the aggressor in the fatal Feb. 26, 2012 confrontation. Zimmerman claims Martin bashed his head into a sidewalk and he shot the teen in self-defense, while prosecutors claim Zimmerman profiled the teen and shot him "because he wanted to."
Before the state rested its case Friday, Martin's mother and brother both testified they believed the voice was Trayvon's.
Responding to questions from defense attorney Mark O'Mara, Donnelly described his experience as a combat medic in Vietnam, saying "you can distinguish the screams for help" of men who are wounded or in life-threatening situations.
"You know, when it's the men you eat with, the men you sleep with, you know who it's going to be before you get there," Donnelly said.
Attorneys played the 911 call in court, a tape Donnelly said was "distressing" to listen to.
"There's absolutely no doubt in my mind that is George Zimmerman," Donnelly said, using a tissue to dab at his eyes. "I wish to God I didn't have that ability to understand that."
Donnelly, who said he thinks of Zimmerman as a "son," said he has contributed money to Zimmerman's defense fund and bought the 29-year-old suits to wear in court.
On cross-examination, Donnelly admitted that combat stress can change voices, but he reiterated that he was able to identify the voices of men he knew even if they were under stress. "I've heard a 250-pound man sound like a little girl screaming, but before you get there, you know who he is," Donnelly said.
Taking the stand Monday, prosecutors played several times a recording of George Zimmerman calling non-emergency dispatchers to report Martin as suspicious person in his community. Prosecutor Bernie de la Rionda asked Leanne Benjamin, Donnelly's wife, whether she thought the profanity-laced terms Zimmerman used in the call demonstrated "ill will" or "spite," elements of the second-degree murder charge Zimmerman faces.
"I have encountered people who use language like that in conversation and it doesn't necessarily come across to me as angry or excited..especially with my kids," Benjamin said.