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SANFORD, Fla. (USA Today) - The mother of teenager Trayvon Martin testified Friday that she believed the screams on a 911 call heard on the night of her son's death were his, not those of George Zimmerman.

"I heard my son screaming," said Sybrina Fulton, who listened to the 911 tape of the final moments of his life on Feb. 26, 2012.

Fulton said she had to listen to the tape only once to know it was her son. She also testified that she didn't think her son was responsible for his own death.

Zimmerman's attorney, Mark O'Mara, pressed Fulton about when she heard the tape and whether she hoped the voice was her son's so she would know the teen hadn't caused his own death. "I didn't hope for anything," Fulton said. "I simply listened to the tape."

Trayvon's older brother, Jahvaris, a 22-year-old student at Florida International University senior, said he also listened to the tape, 10 to 15 times, and said the screams were also made by his brother.

Their testimony could be key in the case against Zimmerman, 29, on trial for second-degree murder for the shooting death of Trayvon, 17. Zimmerman, who has pleaded not guilty, has said that he acted in self-defense after he was attacked.

The Fulton's testimony came on what could be the final day of the state's case against Zimmerman.

State attorneys, who have called three dozen witnesses, have presented a theory that pionts to Zimmerman's motivation: frustrated by neighborhood burglaries, he wrongly profiled an innocent 17-year-old as a criminal, followed the teen, then shot and killed him after a struggle.

Still, despite the prosecution's efforts, several legal experts say the case isn't strong and does not overcome the burden of reasonable doubt.

"There are significant weaknesses in the state's case - most importantly conflicting eyewitness accounts which themselves create reasonable doubt as to what happened that night," said Elizabeth Parker, a former prosecutor who is now a criminal defense attorney in Palm Beach, Fla.

Trayvon's death and the speculation that Zimmerman, who is Hispanic, profiled, followed and murdered him sparked protests around the country last year. Zimmerman, who faces life in prison if convicted, has maintained that race did not factor into his actions.

Prosecutors say they will call the medical examiner who examined Trayvon's body to testify.

Benjamin Crump, the attorney for Trayvon's family, has maintained that prosecutors have enough to convict Zimmerman. "There is overwhelming evidence that George Zimmerman murdered Trayvon Martin," he has said.

Eyewitness accounts concern Parker however, particularly the testimony from residents who lived nearby the shooting scene who told largely different stories.

Jonathan Good said he saw Trayvon on top of and striking Zimmerman moments before the teen was shot.

Selma Mora, who lived a couple of houses down from Good, said a person straddling on top of another person told her to call police. Minutes later, the same person who was on top, Zimmerman, was on his feet after a gunshot, Mora said.

Meanwhile, two key witnesses, Sanford Police Officer Christopher Serino, the lead investigator on the case, and Rachel Jeantel, a young woman who was on the phone with Trayvon moments before he died, did not significantly help the state, Parker said.

Serino agreed with prosecutors that Zimmerman may have been profiling Trayvon, who was black, but also said he believed Zimmerman was telling the truth when he said he was attacked by the teen he fatally shot. The judge later told the jury to disregard that Serino believed Zimmerman.

"Serino's testimony was frankly terrible for the state," said Randy Reep, a Jacksonville defense attorney. "The instruction from the judge to ignore his views of Zimmerman's honesty was the correct thing for the judge to do, but un-ringing that bell is impossible to do."

The investigator also told the jury no physical evidence or witness statements contradicted Zimmerman's claim of self-defense and that the medical examiner's report supported Zimmerman's version of events.

Meanwhile, Jeantel, 19, told jurors that Zimmerman stared at and then followed Trayvon who tried several times to run away. Trayvon was out of breath when he told Jeantel he had lost the man.

"It was kind of strange that a person kept watching and following," she said. "It was like being stalked."

Later, Trayvon told Jeantel the man was back and behind him, she said. Within seconds, she said she heard a "bump" and Trayvon saying "get off." The phone hung up and there was no answer when she called back.

However, Jeantel and defense attorney Don West sparred during a seven-hour cross-examination that was often contentious. Jeantel admitted during testimony that she had lied twice: She said she told Trayvon's family she was 16 when she was 18. She also said she lied about going to a hospital instead of Trayvon's funeral because she didn't want to see his body.

Jeantel also said when she was first interviewed by a state attorney, it was at Trayvon's mother's house. Sybrina Fulton, Trayvon's mother, was crying and Jeantel didn't want to upset her further, the teen said.

"Rachel Jeantel's testimony was crucial to the state's case," Parker said. "She was Trayvon's voice from the grave."

But Parker thought the high school senior came across as aggressive and hostile and that her version of the truth seemed to lack credibility.

The three most troubling issues with Jeantel were that her accounts regarding what she heard were not consistent, her testimony appeared to be inappropriately influenced by the state, and she didn't appear to take testifying seriously, Parker said.

Daryl Parks, another attorney for the family of Trayvon, said his clients were proud of Jeantel and that the teen stuck to what she heard, that Zimmerman was the aggressor.

"She is not a lawyer," Parks said. "She did her very best."

Other state witnesses testified that Trayvon's fingerprints and DNA were not found on the gun but his DNA was found on the right cuff of Zimmerman's jacket. They also said Zimmerman's DNA was not found under Trayvon's fingernails but it was found on a shirt Trayvon wore under his hoodie.

The jury also learned about Zimmerman's rejection letter from a Virginia police department, his course work for a class that discussed stand-your-ground and self-defense laws, and his application to do a police ride-along.

However, legal experts say prosecutors have not proven Zimmerman acted "imminently dangerous" and demonstrated a "depraved mind without regard for human life" - Florida's definition of second-degree murder.

"What they seem to have proved, instead, is a thoughtless, careless encounter that may have spiraled out of control," said Jules Epstein, a Widener University law professor, adding that the prosecution seemed to take an extreme view of the events.

Contributing: Gary Strauss

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