SANFORD -- Trayvon Martin's childhood friend, Rachel Jeantel, was back on the witness stand Thursday morning.

She is one of the prosecution's most important witnesses because she bolsters the contention that George Zimmerman was the aggressor in his confrontation with 17-year-old Martin. She was on the phone with Martin moments before he was fatally shot.

She heard him ask “why are you following me” then “get off! get off!” seconds before Zimmerman fatally shot the Miami Gardens teen last year.

Jeantel, 19, a rising senior at Miami Norland Senior High, testified Wednesday at Zimmerman’s murder trial that she had been talking on a cell phone with Trayvon during his trip to and from a Sanford convenience store the night he was killed.

“He kept complaining that a man was just watching him,” said Jeantel, who had known Trayvon since elementary school.

At some point on Trayvon’s walk back to the home of his father’s fiancée on Feb. 26, 2012, Trayvon told Jeantel that a “creepy-a-- cracker” was following him. She suggested he run, but Trayvon said no.

Jurors appeared transfixed by Jeantel’s testimony, leaning forward and taking rapid notes. Others struggled to decipher the Miami teen’s slang; a court reporter and defense attorney Don West frequently asked Jeantel to slow down, speak up, or repeat what she had said.

The much-anticipated testimony could be key in the case. Prosecutors have tried to depict Zimmerman as an overzealous neighborhood watch volunteer who followed and shot 17-year-old Trayvon, who was unarmed. Defense lawyers have said Trayvon was the aggressor and Zimmerman shot in self-defense as Trayvon hit his head on the sidewalk.

Jeantel, known only as Witness 8 until Wednesday, said Trayvon became alarmed as he tried to lose Zimmerman near the community mailboxes at Retreat at Twin Lakes.

“’Oh, sh--,’” she said Trayvon told her. “’The n---- behind me.’”

The next thing Jeantel said she heard was Trayvon asking the man, “Why are you following me for?”

“Then I heard a hard-breathing man say, ‘What are you doing around here?’”

Jeantel then heard a bump that she assumed was Trayvon’s cell phone headset hitting the ground, followed by the sounds of crumpling grass and Trayvon’s voice: “Get off! Get off!”

Under direct examination, prosecutor Bernie de la Rionda asked if Jeantel attended Trayvon’s funeral and memorial service. She did not, she said, wiping away tears from her eyes, because “I didn’t want to see the body.”

Asked if she’d heard the 911 calls with the audible voice of someone crying out, apparently for help, Jeantel said she had. The panicked voice, she said, “sounds like Trayvon’s.”

On cross-examination, West tried to chip away at Jeantel’s credibility. He pointed out that she initially lied to Trayvon’s mother about her age, saying she was 16 when she was actually 18, and about her reason for missing the funeral, which Jeantel told Sybrina Fulton was because she had been hospitalized.

“You got to understand,” she said to West in one of several tense exchanges. “You’re the last person who talked to a person. You don’t know how I felt. You think I really want to go see the body after I just talked to him?”