The US Department of Justice has announced an investigation into the shooting of an unarmed black teenager in Florida in February.

Trayvon Martin, 17, was killed by neighbourhood watch volunteer George Zimmerman, 28, while walking through a gated community in a suburb of Orlando.

Mr Zimmerman says he was acting in self-defence.

Benjamin Crump, a lawyer for the Martin family, used a news conference on Tuesday to call for his arrest.

A Florida grand jury will also hear evidence in the case on 10 April, a state prosecutor has said.

A grand jury convenes to determine if there is enough evidence to bring a case to trial.

Florida Governor Rick Scott has also asked the Florida Department of Law Enforcement to assist in the investigation.

In an impassioned speech, Mr Crump said police had a clear duty: "Arrest George Zimmerman."

"He is allowed to come and go as he please, while Trayvon Martin is in a grave," Mr Crump said.

According to Mr Crump, a 16-year-old young woman was on the phone with Martin before he was shot. She said Martin was followed by Mr Zimmerman and she heard the beginning of the confrontation, Mr Crump said.

Last phone call

Mr Crump played a recorded affidavit from the young woman in which she describes Martin's last phone call. She spoke with him repeatedly while he visited Sanford, the Orlando suburb where he died.

"She blows Zimmerman's absurd self-defence claim out of the water," Mr Crump said.

According to the affidavit, Martin was walking back from a shop when he ran to a nearby building to take shelter from the rain and then pulled up his hoodie before he walked the rest of the way back. He then realised that someone was following him.

The young woman heard Martin ask Mr Zimmerman repeatedly why he was following him and then described a change in his voice that made her believe he was hit in some way.

She then lost contact with him.

Phone records show the last phone call with Martin started on 19:12 local time on 26 February. Sanford police were on the scene by 19:16, according to a police report on the incident.

"We have all the evidence now," Mr Crump said. He added that three witnesses had come forward to say Martin was the one screaming for help. They could be heard on a separate 911 call.

Mr Crump also questioned why the Sanford police had not tested Mr Zimmerman for drugs or alcohol or completed a background check on him during their investigation.

Outcry

Rallies were held on Monday to demand Mr Zimmerman's arrest and an online petition has more than 500,000 signatories.

Students protested in front of a court building in Sanford, the community where the shooting happened, and on the campus of Florida A&M University in Tallahassee.

Is Florida's self-defence law fair? The incident sheds light on Florida's seven-year-old self-defence law, which critics say is too lenient. The law, nicknamed a "stand your ground" or "shoot first" statute, gives protection from criminal prosecution or civil liability to people who claim self-defence after a shooting or violent incident. One of the most expansive of such laws in the US, it states that people have no duty to retreat from a place they are legally allowed to be, and have the right to use deadly force if they believe they are threatened with death or serious harm.

On Tuesday evening the National Association for the Advancement of Colored People (NAACP) will hold a meeting at a church in Sanford where the first rally for Trayvon Martin was held last week.

More than 500,000 people have signed a global online petition asking for Mr Zimmerman, a neighbourhood watch volunteer, to be prosecuted.

The justice department said in a statement that it would "conduct a thorough and independent review of all of the evidence" before taking "appropriate action".

The FBI and a federal prosecutor in Florida will also assist in the investigation.

It has emerged that Mr Zimmerman, acting as a neighbourhood watch volunteer, had called police several times in the months before the shooting to report incidents.

Call logs and recordings show that Mr Zimmerman called police on 26 February, reporting there had been break-ins in the community.

He told police there was "a real suspicious guy" who "looks like he's up to no good".

When he said he was following the person he had identified as suspicious, the dispatcher said: "We don't need you to do that."

Using a expletive, Mr Zimmerman expressed his frustration, saying "these assholes always get away".

The release of emergency calls recordings, including two from neighbours during which screams and shots can be heard, had fuelled demands for a federal investigation.

The case has focused attention on a 2005 state law which allows deadly force if a person believes their life is in danger.