WASHINGTON -- U.S. President Barack Obama urged Americans to respect the decision of jurors on Sunday following the acquittal of George Zimmerman in the death of unarmed teenager Trayvon Martin, a case that has bitterly divided the country along racial and partisan lines since the boy was gunned down more than a year ago.

"I know this case has elicited strong passions," Obama said in a statement in which he referred to the slaying of the black 17-year-old as a "tragedy" for all of America.

"And in the wake of the verdict, I know those passions may be running even higher. But we are a nation of laws, and a jury has spoken. I now ask every American to respect the call for calm reflection from two parents who lost their young son."

Zimmerman's legal problems, however, seemed far from over a day after a jury of six Florida women cleared him of manslaughter and second-degree murder charges -- a verdict that stunned the black community and civil rights groups and spurred largely peaceful and protests in the streets in several U.S. cities.

In a New York City church, one pastor wore a pink hoodie in memory of Martin on Sunday. The boy was wearing a hooded sweatshirt, common garb among teenagers from all walks of life, when he was killed.

Rev. Jacqueline Lewis urged her congregation to remain peaceful, reminding them that Martin Luther King Jr. "would have wanted us to conduct ourselves on the highest plane of dignity."

But she added: "We're going to raise our voices against the root causes of this kind of tragedy."

The Justice Department said in a statement on Sunday that it's investigating Martin's death to determine whether federal prosecutors should file criminal civil rights charges now that Zimmerman's been acquitted in the state case. The department launched a probe a year ago but stepped aside to allow Florida's prosecution to proceed.

Martin's parents are also considering a wrongful death civil suit against Zimmerman. Sixteen years ago, O.J. Simpson was ordered to pay $25 million in compensation to Nicole Brown Simpson's family members despite his infamous acquittal in criminal court.

Martin was 17 years old and walking to his father's house in Sanford, Fla., carrying a can of iced tea and a box of Skittles, when he was gunned down by Zimmerman, a neighbourhood watch volunteer. Zimmerman claimed he shot the boy in self-defence when Martin began beating on him.

In a country still smarting from a painful history of slavery and segregation, many blacks saw overt racism in Zimmerman's focus on a black boy wearing a hoodie as he walked through a gated community.

The verdict, many argued this weekend, proves the criminal justice system is still inherently biased against black people given Martin, the unarmed slaying victim, was successfully portrayed as the aggressor in the case.

"There may be a civil action brought by the family, but there should definitely be criminal charges brought by (the Department of Justice)," Ben Jealous, president of the NAACP, said Sunday.

"And we have asked D.O.J. to continue their investigation. They are indeed continuing, and we hope that once everything that's happened that can happen here in Florida ... that the D.O.J will act and will hold Mr. Zimmerman accountable for what he has done."

In order to level a hate crime charge, Jealous added, federal investigators need to "show that race was a factor in his decision-making. And there seems to be plenty of evidence that suggests that race may have been a factor. He called 9-1-1 a lot about young black men that he suspected of being dangerous."

Democrats called the outcome a miscarriage of justice.

Republicans felt differently.

"Our justice system is colour-blind," Gov. Rick Perry of Texas said on CNN.

"You had two very capable teams laying out the evidence and the jury made the decision. Though there may be people on either side of this who don't agree with how it came out, we have the very best judicial system."

Iowa congressman Steve King, meantime, said Zimmerman should never have been charged at all.

"I regret that this all happened. I'm sorry that it was turned into a race issue by the media," he said.

Former New York Gov. Eliot Spitzer, a Democrat now running for New York City comptroller, called the verdict a "failure of justice."

"The judicial system is not perfect. And in this case it has failed," he said.