SANFORD, FL (CNN) -- A girl who overheard part of an incident involving Florida teenager Trayvon Martin can help prove he was killed "in cold blood," an attorney for Martin's family said Tuesday.
The girl, who was dating the 17-year-old, "completely blows (George) Zimmerman's absurd self-defense claim out of the water," Benjamin Crump said at a news conference.
He said the girl -- who does not wish to be identified -- "connects the dots" to describe what happened that day.
Martin was shot and killed while walking to his father's fiancee's house in Sanford after a trip to a nearby convenience store February 26. Zimmerman, a neighborhood watch volunteer, has acknowledged shooting Martin.
Although she did not hear the shooting, minutes earlier Martin told her someone was following him, and that he was trying to get away, Crump said.
The girl heard someone ask Martin what he was doing, and heard Martin ask why the person was following him, Crump said. The girl then got the impression that there was an altercation in which the earpiece fell out of Martin's ear and the phone went out, Crump said.
The girl believed that someone had pushed Martin based on what she she heard and the fact that his voice changed.
The girl provided a recording to Crump which he will share with authorities, he said.
However, Crump said the Martin family does not trust the Sanford Police Department to investigate because of "the previous lies that they feel they've been told" by the department.
Federal prosecutors and the FBI will investigate the incident, which has sparked claims of racial profiling and widespread calls for charges to be filed against Zimmerman.
"The department will conduct a thorough and independent review of all of the evidence and take appropriate action at the conclusion of the investigation," Justice Department spokeswoman Xochitl Hinojosa said Monday in a written statement. "The department also is providing assistance to and cooperating with the state officials in their investigation into the incident."
A grand jury will also help investigate the death of the unarmed African-American teenager by Zimmerman.
A police report describes Zimmerman as a white male; his family said he's a Spanish-speaking minority.
The Seminole County Grand Jury will convene April 10, State Attorney Norm Wolfinger said in a statement.
He said he called for an expeditious review from the police department, and "areas for further investigation have been identified."
Police say they have not charged Zimmerman, 28, because they have no evidence to refute his story that the shooting was in self-defense.
In a police report about the incident, Officer Timothy Smith says that when he arrived at the scene, there was a black male lying face down on the ground with his hands underneath his body. "I attempted to get a response from the black male, but was met with negative results," the report says.
Speaking with Zimmerman, who had called 911, Smith observed that Zimmerman's "back appeared to be wet and was covered in grass, as if he had been laying on his back on the ground. Zimmerman was also bleeding from the nose and back of his head."
Smith also wrote that Zimmerman stated, "I was yelling for someone to help me, but no one would help me." That assertion is particularly significant, because Martin's family insists it was Trayvon Martin who was heard yelling for help.
Martin's father said the family believes race was a factor in their son's death, fueling a huge surge in the public outcry over the incident in the racially mixed community 16 miles northeast of Orlando.
"I think that's an issue that Mr. Zimmerman himself considers as someone suspicious -- a black kid with a hoodie on, jeans, tennis shoes," Tracy Martin, the teenager's father, told CNN. "Thousands of people wear that outfit every day, so what was so suspicious about Trayvon that Zimmerman felt as though he had to confront him?"
Zimmerman's family has denied race played any role in the incident. Zimmerman has "many black family members and friends. He would be the last to discriminate for any reason whatsoever," his father, Robert Zimmerman, said in a statement sent to the Orlando Sentinel.
The case was one of the most discussed topics Tuesday morning on Twitter, much of it dedicated to an online petition posted by Trayvon's parents last month calling on Florida authorities to charge Zimmerman.
As of Tuesday afternoon, nearly 584,000 people had signed the petition at Change.org, making it one of the largest petition campaigns ever at the site, according to spokeswoman Megan Lubin. More than 10,000 people an hour were signing the petition early Tuesday.
Demonstrators who have turned out in recent days to protest police handling of the case have mocked Zimmerman's claim, carrying bags of Skittles like the one Trayvon had bought shortly before his death.
CNN has made numerous attempts to contact Zimmerman, but has been unsuccessful.
Zimmerman has moved out of his home after receiving death threats, his father said.
Florida's deadly force law allows people to meet "force with force" if they believe they or someone else is in danger of being seriously harmed by an assailant, but exactly what happened in the moments leading up to Trayvon's death remain unclear.
In his statement last week, Zimmerman's father said his son never followed or confronted Trayvon. But on Monday, police released 911 recordings in which Zimmerman says he is, in fact, following the boy.
"Something's wrong with him. Yep. He's coming to check me out," Zimmerman told a police dispatcher. "He's got something in his hands. I don't know what his deal is. Send officers over here."
The teen started to run, Zimmerman reported. When he said he was following, the dispatcher told him, "We don't need you to do that."
A few minutes later, someone who lives near the scene of the shooting called 911 to report an altercation. In the background, someone can be heard screaming for help, but the caller said she was too afraid to go outside and see what was going on.
Trayvon's father said he believed the pleas for help were his son's last words.
"It's heart-wrenching, because those actually were my son's last words," he said. "And to hear his last words being cries of help, is devastating. It tears me apart as a father."
White House spokesman Jay Carney said Monday that the FBI was monitoring the case but that the White House was not going to "wade into a local law enforcement matter."
Hours later, the Justice Department announced its investigation. It was unclear what prompted the change.
A handful of student protesters and a law professor from Florida A&M University met Monday with a representative of the Seminole County state attorney's office and were told the local investigation will take several weeks, according to Jasmine Rand, the FAMU professor.
Assistant State Attorney Pat Whitaker told the students that the "investigation of the Sanford police needs to be greatly supplemented," according to Reed.
The state attorney's office also said a voice analysis would be conducted on 911 calls from the night of the shooting to determine who was yelling for help, students said.
Florida Gov. Rick Scott weighed in Monday, noting that the case has "caused significant concern within the Sanford community and the state" and asking the Florida Department of Law Enforcement to provide "any assistance necessary" to local investigators.
The Congressional Black Caucus and the Congressional Hispanic Caucus have also called for a federal investigation, with the Black Caucus saying Sanford police showed "blatant disregard for justice" in the case.
In announcing the federal probe, Hinojosa cautioned that bringing a civil rights case requires that the government prove "that a person acted intentionally and with the specific intent to do something which the law forbids -- the highest level of intent in criminal law."
It was unclear when federal officials would announce a decision in the case.
The Justice Department said part of its effort will be to address tensions provoked by the case in the racially mixed community, whose population was 57% white and 30% African-American, according to 2010 U.S. Census Bureau figures. Hispanics, which can be of any race, made up 20% of the population, according to the data.
CNN's Josh Levs, Sunny Hostin, Vivian Kuo, Roland S. Martin, Rick Martin, John Couwels, and Terry Frieden contributed to this report.