SANFORD, Fla. -- An unarmed black teenager shot to death by a neighborhood watch captain told his girlfriend moments before he was killed that he was being followed, a lawyer said Tuesday as federal and state prosecutors announced they would investigate.

"‘Oh he's right behind me, he's right behind me again,'" 17-year-old Trayvon Martin told his girlfriend on his cellphone, the Martin family's attorney said.

The girl later heard Martin say, "Why are you following me?" Another man asked, "‘What are you doing around here?"' attorney Benjamin Crump said.

The phone call that recorded Martin's final moments was disclosed as the U.S. Justice Department opened a federal civil rights probe into the Feb. 26 shooting and the local prosecutor convened a grand jury to investigate. A grand jury will meet April 10 to consider evidence in the case, said Seminole County State Attorney Norm Wolfinger.

The neighborhood watch captain, George Zimmerman, has not been charged and said he shot Martin, who was returning to a gated community in Sanford after buying candy at a convenience store, in self-defense after Martin attacked him. Police say Zimmerman is white; his family says he is Hispanic.

"She absolutely blows Zimmerman's absurd self-defense claim out of the water," Crump said of Martin's girlfriend, whose name was withheld.

The case has ignited racial tensions in this Orlando suburb of 53,500 people, sparking rallies and a protest in Gov. Rick Scott's office Tuesday. The Justice Department's Civil Rights Division said it is sending its community relations service this week to Sanford to "address tension in the community."

At a town-hall meeting Tuesday, more than 350 people packed into the sanctuary of the Allen Chapel AME Church, in a traditionally black neighborhood of Sanford. A line flowed down steps with others trying to get in. The Rev. Al Sharpton was expected to join city leaders at the meeting but did not attend.

Civil rights leaders from the NAACP, American Civil Liberties Union and the Nation of Islam urged residents to remain calm but demand that Zimmerman be arrested. They also said the town's police chief should step down.

"I stand here as a son, father, uncle who is tired of being scared for our boys," said Benjamin Jealous, national president of the NAACP. "I'm tired of telling our young men how they can't dress, where they can't go and how they can't behave."

Residents attending the meeting cheered and jumped to their feet when local NAACP leader Turner Clayton Jr. said the U.S. Department of Justice shouldn't just review the investigation but the federal agency also should take over the Sanford Police Department.

"This is just the beginning of what is taking place," Clayton said. "We're going to make sure justice prevails."

When The Associated Press tried to reach the police department Tuesday evening for comment, a dispatcher told a reporter to call in the morning.

Before the meeting, Sandera Duval held up a white sign in the sanctuary that said in simple black letters, "Justice for Trayvon."

"We want justice for Trayvon because this is a senseless crime," said Duval, 62, a retired nurse. "That could have been my child or my grandchild."

Crump told reporters Tuesday that Martin cried out when a man bearing a 9 mm handgun came at him. Police said Zimmerman, who was found bleeding from his nose and the back of his head, told authorities he yelled out for help before shooting Martin.

Martin, who was in town from Miami to visit his father in Sanford, called his 16-year-old girlfriend in Miami several times on Feb. 26, including just before the shooting, Crump said. The discovery of the lengthy conversations, including one moments before the shooting, was made over the weekend by Martin's father, who checked his son's cellphone log, Crump said.

The teenager told the girl on his way back from the store he'd taken shelter from the rain briefly at an apartment building in his father's gated community, Crump said. Martin then told her he was being followed and would try to lose the person, Crump said.

"She says: `Run.' He says, `I'm not going to run, I'm just going to walk fast,"' Crump said, quoting the girl.

After Martin encountered Zimmerman, the girl thought she heard a scuffle "because his voice changes like something interrupted his speech," Crump said. The phone call ended before the girl heard gunshots.

The last call was at 7:12 p.m. Police arrived at 7:17 p.m. to find Martin lying face down on the ground.

Zimmerman was handcuffed after police arrived and taken into custody for questioning, but was released by police without being charged. Police have interviewed Zimmerman two times since then.

Crump called the treatment patently unfair and asked if Martin would have received the same treatment if he had been the shooter.

"We will not rest until he is arrested. The more time that passes, this is going to be swept under the rug," Crump said.

Crump said he plans to turn over information about the call to federal investigators; a grand jury in Seminole County is also likely to subpoena the records. The Florida Department of Law Enforcement is also involved in the state case.

Former federal prosecutors said there are limitations to a Justice Department civil rights probe, which typically would involve a sworn law enforcement officer accused of abusing his authority.

In this case, they said, it's not clear whether Zimmerman had any actual law enforcement authority or if the Sanford Police Department did anything improper. Zimmerman had a permit to carry a gun, but it was not required for his neighborhood watch patrol.

"I think the community has the feeling that there's some type of cover-up," said Jeffrey Sloman, former U.S. attorney in Miami. "At least the department's involvement makes sure it gets some review. He wasn't a police officer. I'm sure that this is going to be a tough case to prosecute."

Authorities may be hamstrung by a state "Stand Your Ground" law that allows people to defend themselves with deadly force and does not require a retreat in the face of danger. Asked Tuesday if that law needs change, Republican Gov. Rick Scott said "it's always positive to go back and think about existing laws."

During the town hall meeting in Sanford, Florida Rep. Geraldine Thompson promised the law's repeal would be a top priority for the state legislature's black caucus.

"If vigilante justice becomes the norm, will visitors feel comfortable coming to our state?" she said.

An online petition urging local authorities to prosecute Zimmerman has drawn more than 500,000 signatures at website Change.org. About 50 defense attorneys and protesters filled the lobby in the governor's office Tuesday to deliver a letter seeking an independent investigation and a task force to study racial profiling. They applauded when Scott came out of his office to talk to them.

"I will make sure justice prevails," Scott said. "I'm very comfortable that (state law enforcement) is going to do the right thing. They're not going to let somebody do something wrong and get away with it."

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Anderson reported from Fort Lauderdale, Fla. Associated Press writers Brendan Farrington and Brent Kallestad contributed from Tallahassee.