SANFORD, Fla. (NBC News) -- It's been another explosive day of testimony in the George Zimmerman murder trial.
This afternoon prosecutors called to the witness stand, the teenager Martin was talking with on the phone just before he was killed.
Rachel Jeantel told the court she was on the phone with Wartin when he told her he was being followed.
"He said the man started following him," Jeantel testified.
Next she said she heard what sounded like a struggle.
"I heard wet grass, I heard Trayvon saying get-off, get-off," said Jeantel.
Prosecutors then asked Jeantel if she had heard the 9-1-1 call from that night in which someone can be heard screaming then the sound of gunshots.
Jeantel said she had heard the call and recognized the voice, telling the court it was Travon's.
It's a question at the core of the case.
Who is screaming for help in the background of that call.
George Zimmerman told police he was the one yelling that night.
But earlier witness Jane Surdyka, who also called police that night told the jury that while watching and hearing things unfold from her second floor window that she believes it was the teenager she heard screaming for help.
"I truly believe, especially the second yell for help that was like a yelp, it was excruciating, i really felt it was the boy's voice," Surdyka said.
During cross examination the defense pushed back, getting Surdyka to admit that she wasn't sure whose voice she heard.
Ultimately the jury will have to decide, among other issues, whose muffled voice was calling for help that night.
-- Jay Gray, NBC News, reports