Trayvon Martin: Obama says teenager's death a tragedy
US President Barack Obama has said the "tragedy" of an unarmed black teenager shot dead in Florida should prompt some national soul-searching.
The death of Trayvon Martin, 17, gunned down by a neighbourhood watchman, who was not charged as he claimed self-defence, has sparked outrage.
"If I had a son he would look like Trayvon," President Obama told reporters at the White House.
Rallies have been held this week in Florida and New York to demand justice.
"I can only imagine what these parents are going through and when I think about this boy I think about my own kids," Mr Obama said on Friday.
"I think they are right to expect that all of us as Americans take this with the seriousness that it deserves and we're going to get to the bottom of what happened.
"Every parent in America should be able to understand why it is absolutely imperative that we investigate every aspect of this and everybody pulls together, federal state and local, to figure out exactly how this tragedy happened," he added.
President Obama's remarks were echoed by Republican White House hopefuls who spoke out later on Friday about the case.
Former Massachusetts Governor Mitt Romney said in a statement: "What happened to Trayvon Martin is a tragedy. There needs to be a thorough investigation that reassures the public that justice is carried out with impartiality and integrity."
His rival, former Pennsylvania Senator Rick Santorum, called the shooting a "horrible case" and disputed whether Mr Zimmerman deserved protection under the state's self-defence law.
"Stand your ground is not doing what this man did," he said.
In Florida, a law known as "stand your ground" can prevent criminal or civil prosecution when deadly force is used in self-defence.
There have been mounting calls for the arrest of George Zimmerman, 28, who opened fire on the teenager on 26 February in the Orlando suburb of Sanford.
A grand jury is considering whether to charge Mr Zimmerman and will hear evidence on the case on 10 April.
The Department of Justice and FBI have launched a civil rights investigation into the conduct of the local police department.
Mr Martin was carrying a bag of sweets and a can of iced tea when he was approached by Mr Zimmerman.
The neighbourhood watchman had told a police dispatcher he thought Mr Martin looked suspicious.
It was raining and the teenager had his jacket hood pulled over his head. Mr Zimmerman shot Mr Martin following a confrontation.
The Florida politician behind the state's 2005 "stand your ground" law has said inan opinion piece for Fox Newsthat he did not believe the rule was applicable in the case of Mr Martin.
The law "does not provide protection to individuals who seek to pursue and confront others, as is allegedly the case in the Trayvon Martin tragedy in Sanford", said Republican State Representative Dennis Baxley.
On Thursday, Florida Governor Rick Scott appointed a new prosecutor to handle the case.
He also established a task force led by the state's lieutenant governor, an African-American woman, to conduct hearings on the incident and recommend any changes to state law.
Hours earlier, Sanford Police Chief Bill Lee temporarily stepped down in a bid to defuse building anger that his department had not arrested Mr Zimmerman.
Thousands of people gathered at a rally in Sanford on Thursday evening led by the civil rights leader Al Sharpton demanding an arrest over the 17-year-old's shooting.
More than 1.5 million signatures have been gathered for an online petition calling for justice for Mr Martin.
Mr Obama, the first US African-American president, has largely steered clear of race issues since his much-criticised intervention in the controversial 2009 arrest of a black Harvard University professor.
The president said police had "acted stupidly" in detaining the academic as a suspected burglar, but later added it would have been preferable if he expressed his concerns in different terms.