A grand jury will hear evidence next month in the incident involving an unarmed black teenager fatally shot by a neighborhood watch captain in Florida.

The news comes after new details from the shooting emerged Tuesday. The attorney for the family of the 17-year-old victim, Trayvon Martin, said that the boy was talking to his girlfriend on his cell phone when the confrontation began. She heard an exchange of words, what sounded like a scuffle but not the shooting.

The phone call was discussed at a news conference that followed announcements that the U.S. Justice department would probe the death of Martin and that a local grand jury will also consider evidence in the case.

The episode is scheduled to be examined on April 10, according to the state attorney for Brevard and Seminole counties, Norman R.Wolfinger, who has asked for the public’s patience as the investigation continues.

George Zimmerman, 28, claims he shot Martin in self-defense during the confrontation last month in a gated community in Sanford, Fla. Zimmerman spotted Martin as he was patrolling his neighborhood on a rainy evening and called 911 to report a suspicious person. Against the advice of the 911 dispatcher, Zimmerman then followed Martin, who was walking home from a convenience store with a bag of Skittles in his pocket.

Police have described Zimmerman as white. His family says he is Hispanic and not racist.

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Attorney Benjamin Crump, who represents Martin's parents, said the teenager was on the phone with his girlfriend back home in Miami after he left the store. Martin told the girl he'd taken shelter from the rain briefly at an apartment building in the gated community before continuing his walk to where he was staying with his father nearby.

It was then that Martin told the girl he was being followed, according to Crump. She said Martin told her someone was following him and that he was going to try to lose him. He thought he had lost Zimmerman but hadn't.

"He says, 'Oh he's right behind me, he's right behind me again,'" Crump says the girl told him. "She says: 'Run.' He says, 'I'm not going to run I'm just going to walk fast.' She hears Trayvon say, 'Why are you following me?' Other voice says, 'What are you doing around here?'"

She told Crump they both repeated themselves and then she thinks she heard Zimmerman push Martin "because his voice changes like something interrupted his speech." She heard an altercation and then the phone call was cut off.

Within moments, according to Crump's timeline, Martin was shot. She didn't hear the gunfire.

Crump is not releasing the girl's name to protect her privacy.

Crump said he plans to turn over information about the call to federal investigators who are looking into the case. The Justice Department announced their involvement late Monday.

"The department will conduct a thorough and independent review of all the evidence and take appropriate action at the conclusion of the investigation," the agency said in an emailed statement.

The federal agency said it is sending its community relations service this week to Sanford to meet with authorities, community officials and civil rights leaders "to address tension in the community."

An online petition urging local authorities to prosecute Zimmerman has drawn more than 500,000 signatures at website Change.org.

Later Tuesday, civil rights activist Al Sharpton is expected to join Sanford city leaders in an evening town hall meeting to discuss with residents how the investigation is being handled. On Monday, students held rallies on the campus of Florida A&M University in Tallahassee and outside the Seminole County Criminal Justice Center, where prosecutors are reviewing the case to determine if charges should be filed.

Authorities may be hamstrung by a state law that allows people to defend themselves with deadly force.

The law was changed in 2005. Before, people could use deadly force in self-defense only if they had tried to run away or otherwise avoid the danger.

Under the new law, there is no duty to retreat and it gives a Floridian the right "to stand his or her ground and meet force with force, including deadly force," if he feels threatened.

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On Monday, about 70 protesters at the rally in Sanford called for Zimmerman's arrest. They chanted "What if it was your son?" and held posters saying, "This is not a race issue."

"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, said at the Sanford rally.

U.S. Rep. Corrine Brown, D-Fla., along with members of the Congressional Black Caucus and the Congressional Hispanic Caucus, had asked the U.S. Department of Justice to review the case. Late Monday, Florida Gov. Rick Scott also directed the state Department of Law Enforcement to help local authorities in their investigation.

Prosecutors can have a hard time making a case if there is no one else around to contradict a person who claims self-defense, said David Hill, a criminal defense attorney in Orlando. So far, Sanford police have said there is no evidence to contradict Zimmerman's claims.

Gun control advocates said the case is emblematic of permissive gun laws in Florida, which was among the first states to allow residents to carry concealed weapons. Florida was the first state to pass a "Stand Your Ground" law, which has been dubbed a "Shoot First" law by gun control advocates.

Currently, about half of all U.S. states have similar laws, said Brian Malte, legislative director of the Brady Campaign, which describes itself as the nation's largest organization dedicated to the prevention of gun violence.

"It's coming to dangerous fruition," Malte said. "There are more states like Florida."

The "Stand Your Ground" law's legislative sponsor, Florida Rep. Dennis Baxley, said it wasn't written to give people the power to pursue and confront others.

Based on reporting by the Associated Press.

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