New details are emerging about the slaying of Trayvon Martin, a black Florida teenager whose case has stirred national outrage and protests.

An undated file family photo of Trayvon Martin. (AP)

The teenage girl, who requested that her name not be used, told ABC of her call with Martin:

“He said this man was watching him, so he put his hoodie on. He said he lost the man... [I] asked Trayvon to run, and he said he was going to walk fast. I told him to run but he said he was not going to run.”

Martin, who was unarmed, did eventually run, but was cornered by the “strange man,” according to the girl. She told ABC:

“Trayvon said, ‘What, are you following me for,’ and the man said, ‘What are you doing here.’ Next thing I hear is somebody pushing, and somebody pushed Trayvon because the head set just fell.”

After that, she said, the line went dead. She says she tried to called her friend again and he didn't answer the phone.

Martin’s case has stirred the anger of students, African American leaders and others who say the local Florida police department has been slow to act. Online, the outrage is visible on Facebook pages and on Twitter. A petition by Martin’s parents to bring Zimmerman to justice has more than 500,000 signatures; another petition urging for stronger gun laws in Florida has more than 200,000.

A rally demanding justice for Trayvon Martin marches toward the Titusville, Fla. courthouse on Sunday. (Craig Rubadoux/AP)

On Monday, Federal authorities announced they would open a full-scale criminal investigation into his death, in an attempt to assuage the rising tensions.

Much of the outrage is centered around 911 tapes released last week that suggest, as ABC’s account does, that Zimmerman may have pursued Martin during the incident on Feb. 26.

Zimmerman first called 911 from his car, after he saw Martin walking through the neighborhood. “We’ve had some break-ins in the neighborhood, and there’s this real suspicious guy,” he told the dispatcher. “This guy looks like he’s up to no good or he’s on drugs or something. It’s raining and he’s just walking around, looking about.”

Mother Jones has transcribed the entire 911 call here

Those arguing that the killing may have been racially-charged are pointing to one line in the 911 call, in which Zimmerman whispers an expletive followed by something unintelligible. Some listeners have suggested that the unintelligible word is a racial slur, while others say he said “punks,” “course” or something else altogether. You can listen to the line here.

Zimmerman mentions Martin's race numerous times in his call, saying at one point: “He's got his hand in his waistband... and he's a black male.”

While it is common for 911 dispatchers to ask the race of a potential aggressor for identification purposes, New York Times columnist Nick Kristof says he thinks the way in which race was discussed in the call had “shades of 1950's Mississippi.”

At the end of the call, Zimmerman says the teenager is running away, and the dispatcher warns Zimmerman not to follow him, saying an officer is on the way.

Moments later, Martin was killed by a bullet to the chest.

A request for comment from the Sanford Florida police department about the 911 call was not immediately returned.

In a statement released to the Orlando Sentinel, Zimmerman’s father says his son did not try to pursue or confront Martin, and did not target him because he was black. Zimmerman says he shot Martin in self-defense.

According to Florida self-defense laws:

“There must be an overt act by the person which indicates that he immediately intends to carry out the threat. The person threatened must reasonably believe that he will be killed or suffer serious bodily harm if he does not immediately take the life of his adversary.”

“I don’t think a man who exited his vehicle after the 911 dispatcher told him to stay inside the car can claim self-defense,” Carl McPhail, a 28-year-old Barry University law school student, told the Associated Press.

Sanford is a city that has struggled with racial tensions for a century, according to McClatchy/Tribune wire services.