SANFORD, Fla. -

Prosecutors in the George Zimmerman trial called Trayvon Martin's mom Sybrina Fulton to the witness stand Friday morning.

Prosecutor Bernie de la Rionda played the chilling 911 call that recorded a person screaming for help on Feb. 26, 2012. Fulton maintained her composure as she listened.

"I heard my son screaming," said Fulton, who has worked for Miami-Dade County for more than two decades.

FBI forensics expert Hirotaka Nakasone testified earlier in the trial that humans familiar with a voice could be reliable sources of identification.

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"My youngest son is Trayvon Benjamin Martin," Fulton said with conviction. "He's in heaven."

She said 17-year-old Trayvon had two tattoos. He had a tattoo of praying hands on his right arm and her name on his left wrist.

When defense attorney Mark O'Mara cross examined her,  she said Trayvon's dad, Tracy Martin, did not tell her he had already listened to the 911 recording. Martin had concluded Trayvon was not the one screaming and later changed his mind.

When O'Mara tried to get Fulton to say that her hopes as a mother were clouding her judgment, Fulton was assertive.

"What I hope for is that this would not have happened and he still would be here," Fulton said.

Fulton did not cry during testimony.

Before entering the courtroom, Fulton tweeted: " I pray that God gives me strength to properly represent my Angel Trayvon."

Her son Jahvaris Fulton also testified that he recognized the screams as that of his brother's Trayvon.

After their testimony, Fulton tweeted a picture: "... and I still have and love my 2 Boys."