SANFORD, Fla. – A Vietnam combat veteran and close friend of George Zimmerman testified Monday that war gave him a unique ability to identify screams, and that the screams for help heard on a crucial 911 call “definitely” belonged to Zimmerman.

“In the midst of combat, there are a lot of people yelling,” said John Donnelly, who served as an Army combat medic. “I have heard a 250 pound man scream like a little girl,” John Donnelly said.

Asked by Zimmerman attorney Mark O’Mara if he had ever experienced treating injured fellow soldiers that he had known, Donnelly, who was at times emotional during his testimony, said “Yes,” and said “It’s a little difficult” to talk about.

After hearing the 911 call, Donnelly said “there’s absolutely no doubt in my mind that it’s George Zimmerman” screaming.

“This courtroom is about truth,” he said later.

Donnelly is one of several defense witnesses who have already testified that the screams came from Zimmerman. Judge Debra Nelson, who is presiding over the trial, earlier ruled out the possibility of hearing testimony from voice identification experts.

Donnelly and his wife Leanne Benjamin, who testified earlier in the morning, have given $3,000 to Zimmerman for his defense fund. Donnelly also said that he bought $1,700 worth of clothes and ties for Zimmerman for the trial. To do so was to show “respect for the process,” he said.

“I think of him as a son,” said Donnelly.