The civil rights division, with the FBI, will attempt to establish how detectives concluded that a slight, 17-year-old boy represented a mortal threat to his stocky killer.
Martin's parents are doggedly campaigning for a proper investigation into the shooting. His mother, Sybrina Fulton, appeared on television on Tuesday to allege that her son was killed because Zimmerman was: "reacting to the colour of his skin".
Lawyer Benjamin Crump, who represents Martin's parents, said the teenager was on the phone with his girlfriend when he told her he was being followed. Martin told the girl he'd taken shelter from the rain briefly at an apartment building before continuing his walk to where he was staying with his father nearby.
"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. She didn't hear the gunfire.
Police say Zimmerman was bleeding from his nose and the back of his head, and told police he had yelled out for help before he shot Martin.
Crump said yesterday that it was Martin who cried out when a man bearing a 9mm handgun came at him.
"She absolutely blows Zimmerman's absurd self-defence claim out of the water," Crump said of Martin's girlfriend, whose name was withheld.
Martin called his 16-year-old girlfriend in Miami several times on February 27, Crump said.
Martin's father discovered the lengthy conversations by checking his son's cellphone log, Crump said.
The last call was at 7.12pm local time. Police arrived at 7.17pm 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.
"We will not rest until he is arrested," Crump said. "The more time that passes, this is going to be swept under the rug."
The lawyer said he planned to turn over information about the call to federal investigators; a grand jury is also likely to subpoena the records. The Florida Department of Law Enforcement is also involved in the state case.
The case has ignited a furore against the local police department of this Orlando suburb of 53,500 people, sparking rallies and a protest in Governor Rick Scott's office yesterday.
The Justice Department's Civil Rights Division plans to send its community relations service to Sanford to "address tension in the community".
At a town hall meeting, more than 350 people packed into the Allen Chapel AME Church in a traditionally black neighbourhood of Sanford.
Civil rights leaders urged residents to remain calm but demand Zimmerman be arrested. They also said the town's police chief should step down.
Authorities may be limited by a state law that allows people to defend themselves with deadly force.
Florida was the first state to pass in 2005 a "Stand Your Ground" law, which has been dubbed a "Shoot First" law by gun control advocates.
About half of all US states now had similar laws, said Brian Malte, legislative director of the Brady Campaign, described as the nation's largest organisation dedicated to the prevention of gun violence.
'THIS GUY LOOKS LIKE HE'S UP TO NO GOOD'
George Zimmerman had followed Trayvon Martin despite being advised not to by a police operator. Zimmerman can be heard on emergency tape calling police to report a suspicious character where he lived.
"This guy looks like he's up to no good, or he's on drugs or something," he tells the dispatcher.
"These arseholes, they always get away," he adds.
Zimmerman tells the operator that he is following Martin, after which the operator tells him not to do so.
A second 911 call, this time from a neighbour who witnessed a struggle between the two men, could also prove crucial to any future investigation.
The neighbour says she can hear "someone screaming outside".
"I don't know why. I think they're yelling help but I don't know," she says.
The sound of a struggle can be heard, followed by gunshots.
- AP, Independent
