Federal authorities announced Monday night that they are opening a full-scale criminal investigation into the slaying of an unarmed black Florida teenager whose death provoked an outcry from African American leaders and sparked calls for gun-control reforms in Florida.

A Justice Department spokeswoman said in a statement that its Civil Rights Division, in conjunction with the FBI, planned to conduct an investigation into the death last month of Trayvon Martin and to work with local groups in the Orlando area to soothe rising tensions over the matter.

“The department will conduct a thorough and independent review of all of the evidence and take appropriate action at the conclusion of the investigation,” spokeswoman Xochitl Hinojosa said in a statement. “The department also is providing assistance to and cooperating with the state officials in their investigation into the incident.”

National figures such as the Rev. Al Sharpton and members of the Congressional Black Caucus have charged that local authorities have failed to deliver justice for the family of Martin, who was shot by a neighborhood watch volunteer. The volunteer told a police dispatcher that the 17-year-old appeared suspicious.

“I am outraged by the way in which this case has been handled by the Sanford Police Department in Florida,” said Rep. Emanuel Cleaver II (D-Mo.), chairman of the Congressional Black Caucus, in a statement issued Monday before the Justice Department announced its investigation. “Those who are meant to protect us and our children have blatantly turned their backs on fairness and justice.”

The demands for a federal probe follow weeks of mounting criticism aimed at the Sanford Police Department, particularly over its reluctance to charge or arrest George Zimmerman, 28, who told police he shot Martin in self-defense.

Officials with the department have said in public statements that they lack sufficient evidence to disprove Zimmerman’s accounting of the events and have passed the case on to the state’s attorney. Neither office returned phone calls from The Post requesting comment Monday.

The criticism exploded into outrage last week after the department released tapes of 911 calls that suggested Zimmerman may have pursued the teenager during the incident Feb. 26.

The case has also drawn new scrutiny to Florida’s “Stand Your Ground” law, a measure backed by the National Rifle Association and adopted in 2005, which expanded the rights of citizens to claim self-defense in killings. More than 20 states have similar laws on the books, according to the Brady Campaign to Prevent Gun Violence.

According to the tapes, Zimmerman called 911 from his car after spotting Martin walking through the neighborhood. “We’ve had some break-ins in the neighborhood, and there’s this real suspicious guy,” he told the dispatcher. “This guy looks like he’s up to no good or he’s on drugs or something. It’s raining and he’s just walking around, looking about.”

Later in the call, Zimmerman said the teenager was running away. The dispatcher advised Zimmerman not to follow and said that an officer was on the way. Minutes later, Martin was killed by a bullet to the chest, which Zimmerman said he discharged in self-defense.

Last week, Zimmerman’s father released a statement to the Orlando Sentinel saying that his son did not try to pursue or confront Martin. And he disputed allegations that his son targeted Martin because he was black.

“George is a Spanish-speaking minority with many black family members and friends,” Robert Zimmerman wrote. “He would be the last to discriminate for any reason whatsoever.”

Benjamin Crump, an attorney for Martin’s parents, said the teenager had not been using drugs or alcohol and would have had no reason to confront Zimmerman. Martin had been watching the National Basketball Association All-Star Game at his father’s girlfriend’s house in the neighborhood when he ventured out to a nearby 7-Eleven store for a snack, Crump said. A bag of Skittles and a can of iced tea were on his body when police arrived on the scene.

After days went by without an arrest, Crump said, he decided to reach out to national figures such as Sharpton. The civil rights leader and MSNBC commentator plans to appear at a rally in Sanford on Thursday and will broadcast his television program from the scene. In an interview, Sharpton said he hopes not only to draw attention to Martin’s case but to the “Stand Your Ground” law, which critics say allows killers to argue in court that their actions were justified because they felt threatened.

“We are asking the Justice Department to examine that law and challenge whether it violates civil rights,” Sharpton said.

Staff writer Jerry Markon and staff researcher Lucy Shackelford contributed to this report.