The decision by the Justice Department and the FBI to open an investigation into the slaying of an unarmed black teenager in Florida has spurred internal debate at the agency over whether the federal government could bring criminal charges in the case, which has sparked widespread protest.

Lawyers at the department said Tuesday that while the investigation into the shooting of 17-year-old Trayvon Martin would go forward, it would be difficult to prosecute the case under federal law. Civil rights law protects against “hate crimes” or actions by police officers, but Martin’s shooting may not have either of those elements, two officials said. They spoke on the condition of anonymity because the probe is still under federal review.

Martin was shot and killed Feb. 26 by a neighborhood watch volunteer, George Zimmerman, who told police he was acting in self-defense. Zimmerman, 28, had called police from his car after he saw Martin walking in a gated community in Sanford, Fla.

According to the 911 tapes, Zimmerman told the dispatcher, “this guy looks like he’s up to no good or he’s on drugs or something . . . they always get away.” The dispatcher told Zimmerman not to follow, saying an officer was on the way. Minutes later, Martin was shot in the chest.

No charges have been brought against Zimmerman. Along with the Justice investigation, a local grand jury will consider evidence in the case.

Zimmerman’s family described him as “a Spanish-speaking minority,” and his father released a statement to the Orlando Sentinel saying his son did not target Martin because he was black.

Martin’s mother, Sybrina Fulton, said Monday, “I don’t understand why this man has not been arrested . . . let a judge and jury decide if he’s guilty.”

Lawyer Benjamin Crump, who represents Martin’s parents, said at a news conference in Florida on Tuesday that the teenager was on a cellphone with his girlfriend in Miami when he told her he was being followed, according to the Associated Press. She said Martin told her that he was trying to get away.

Crump, who did not release the name of Martin’s girlfriend because of privacy concerns, said she heard a scuffle and an altercation before the call was cut off.

Martin had not been using drugs or alcohol and would have had no reason to confront Zimmerman, Crump said in an interview. He had been watching basketball at his father’s girlfriend’s house when he went 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.

Rallies have been held across Florida, with students calling for Zimmerman’s arrest, and the Rev. Al Sharpton will hold a national rally Thursday in Sanford.

Thomas E. Perez, assistant attorney general for civil rights, met in Washington on Tuesday with Sanford Mayor Jeff Triplett and Rep. Corrine Brown (D-Fla). Brown urged Perez to prosecute the case as a hate crime.

Stephen A. Saltzburg, a professor at George Washington University Law School, called the case “a difficult one for the Justice Department.”

“This may be somebody who is racially biased, but from the 911 calls, it looks as though, however misguided this guy was, he thought that Trayvon was involved in some kind of suspicious activity,” Saltzburg said. “Race may play a role, but I just think it will be hard to bring this as a federal hate crime, given the limited reach of federal hate-crimes law.”

Justice Department spokeswoman Xochitl Hinojosa issued a statement late Monday saying that in civil rights crimes, the government “must prove beyond a reasonable doubt that a person acted intentionally and with the specific intent to do something which law forbids — the highest level of intent in criminal law.”

White House spokesman Jay Carney offered condolences to the Martin family but said the White House was “not going to wade into a local law enforcement matter.”

Staff writer Krissah Thompson and staff researcher Julie Tate contributed to this report.