George Zimmerman trial defense attorneys called Mark Osterman to the witness stand Monday morning.

Defense attorney Mark O'Mara highlighted Osterman's law enforcement experience as a federal air marshal.  Osterman was the friend who recommended Zimmerman buy a Kel-Tec 9 mm handgun, because of its "safety features."

"It's always better to need it and have it, than to need it and not have it," Osterman said.

On Feb. 26, 2012, Zimmerman used the gun to kill Trayvon Martin and claims he killed in self-defense. 

Just as he did with witnesses Gladys Zimmerman, Jose Meza and Sandra Osterman, O'Mara played the 911 call recording of the screamer.

"I thought it was George," Osterman said.  "Just the tone, the volume and the tone, because I talk to him probably as much on the phone."

Osterman told prosecutor Bernie de la Rionda that he helped Zimmerman at the shooting range.

On July 2nd as a prosecution witness, Osterman said Zimmerman was the best friend he had ever had. He said he had known him for at least five years.

Osterman co-authored a book titled Defending our Friend: The Most Hated Man in America based on what Zimmerman told him happened the Feb. 26, 2012 night he killed Trayvon.

Osterman said he picked up Zimmerman from the Sanford Police Department station after the fatal shooting.  

Prosecutor Bernie de la Rionda also questioned Osterman on what Zimmerman told him. Osterman said Zimmerman told him Trayvon tried to grab his gun.

"I thought he had said he grabbed the gun," Osterman said.

Zimmerman told police he thought Trayvon was reaching for his gun -- not grabbing it.

Osterman also wrote in his book that Zimmerman pinned Trayvon’s hands down.  Police found Trayvon’s hands under his body. When De la Rionda mentioned it, Osterman claimed he did not know that.