The screams on the 911 call played over and over Monday for jurors here are chilling.

“Help!” someone cries. “Help!”

Identifying that person has become an urgent fixation for lawyers in the trial of George Zimmerman, the former neighborhood watch volunteer accused of second-degree murder in the fatal shooting of an unarmed African American teenager named Trayvon Martin. The defense’s assertion that Zimmerman is screaming for help is key to his self-defense argument; prosecutors want jurors to believe that Martin is screaming for help because he is being attacked by Zimmerman.

On Monday afternoon, two Sanford police investigators — Chris Serino and Doris Singleton — gave a possible boost to the defense by testifying that Martin’s father, Tracy Martin, told them that the voice was not his son’s. “There’s no doubt” about what Tracy Martin concluded after officers played the tape for him, Singleton testified. “He was crying. He was sad.”

But Tracy Martin provided a potentially powerful counterweight, leaving his customary seat in the second row of the courtroom audience and testifying that he told the police officers that he could not identify the screamer. He told jurors that after talking to police he listened to the tape 20 times and decided his son was the person screaming. “I was listening to my son’s last cry for help,” he said.

Last week, Trayvon Martin’s brother, Jahvaris Fulton, and his mother, Sybrina Fulton, were called by prosecutors to testify that the screams belong to Martin. Defense lawyers attempted to neutralize the testimony by Martin’s family members by having Zimmerman’s mother, Gladys Zimmerman, testify that the voice on the tape belonged to her son, who was 27 at the time of the shooting.

On Monday, defense lawyers flooded the courtroom with the testimony of witnesses who said Zimmerman can be heard screaming in the background of the 911 call placed by a neighbor. The six-woman jury heard from two couples — Mark and Sondra Osterman, and Leanne Benjamin and John Donnelly — as well as one of Zimmerman’s work colleagues, Geri Russo.

“Definitely, it’s Georgie,” said Sondra Osterman, using her nickname for the man who her husband testified earlier in the trial is the “best friend” he ever had.

Defense witnesses also sought to humanize Zimmerman, whose trial entered its 10th day of testimony Monday after prosecutors rested their case Friday. He aspired to open his own business, Benjamin said, then he wanted to go to college to focus on legal studies. He needed help tying a Windsor knot, Donnelly told jurors.

The Ostermans played a particularly important role in Zimmerman’s life and in the sequence of events that culminated with him driving through the streets of his community on the night of Martin’s death wearing a holstered Kel-Tec 9mm pistol. Mark Osterman

and Zimmerman went to shooting ranges together, he said. It was Osterman who suggested that Zimmerman keep a bullet in the chamber, as well as a full magazine, so that the gun could be fired more quickly.

Donnelly, who says he thinks of Zimmerman almost “as a son,” contributed at least $2,500 for Zimmerman’s legal expenses and $500 to a personal Web site Zimmerman set up. But the man who taught his younger friend how to tie a Windsor knot also made another contribution: He took Zimmerman to Men’s Wearhouse to buy $1,700 worth of suits and ties for the trial.

“You got a good deal?” prosecutor Bernie de la Rionda asked.

“Yes, sir,” Donnelly said with a smile. “They were on sale.”