Sanford, Florida (CNN) — Federal prosecutors and the FBI have opened an investigation into the killing of an unarmed teen by a neighborhood watch volunteer in Florida.

The death drew protesters Monday to the courthouse in Seminole County, north of Orlando, to demand justice for Trayvon Martin, the 17-year-old who was shot and killed last month while walking back to his father’s fiancee’s house in Sanford.

And it drew calls for a federal investigation from the Congressional Black Caucus, saying local police have shown “blatant disregard for justice.”

Late Monday, the Justice Department said it would dispatch officials to Sanford to investigate and “to address tension in the community.”

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

Florida Gov. Rick Scott also weighed in Monday evening, ordering the state Department of Law Enforcement to provide “any assistance necessary” to local investigators.

“No justice for Trayvon. No peace for Sanford!” read another.

At the Seminole County courthouse, a handful of student protesters and a law professor from Florida A&M University met with a representative of the state attorney’s office to discuss the ongoing investigation while protesters demanded the arrest of the neighborhood watch captain, George Zimmerman. Many carried signs in remembrance of Martin.

“Gone but not forgotten,” read one that had a picture of the young man wearing a football uniform. “No justice for Trayvon. No peace for Sanford!” read another.

Assistant State Attorney Pat Whitaker told the students it would take several weeks to look at the case, but that the “investigation of the Sanford police needs to be greatly supplemented,” Jasmine Rand, the FAMU professor, said after the meeting.

The state attorney’s office also said a voice analysis would be conducted on 911 calls from the night of the shooting to determine who was yelling for help, students said.

A spokeswoman for the state attorney’s office confirmed Whitaker met with the group, but declined to comment on the specifics of what was said.

According to the 911 calls released on Friday, terrified neighbors implored dispatchers to send police as a voice in the background screamed for help.

Martin was carrying a drink and candy when Zimmerman called 911 to report a suspicious man, authorities said.

The 911 dispatcher told Zimmerman not to confront him. But by the time police arrived, Martin lay dead with a gunshot wound in the chest, according to Sanford Police Chief Bill Lee.

Zimmerman told police he shot the teen in self-defense, authorities said, and remains free as the state attorney investigates. Police said he has not been charged because there are no grounds to disprove his story of what happened.

“The evidence and testimony we have so far does not establish that Mr. Zimmerman did not act in self-defense,” the police chief said. “We don’t have anything to dispute his claim of self-defense, at this point, with the evidence and testimony that we have.”

“It’s surprising. It’s shocking,” said Tracy Martin, the victim’s father. “It lets me know that justice is just not being served here. All we want is justice for our son. We’re not asking for anything out of the ordinary.”

CNN has made numerous unsuccessful attempts to contact Zimmerman. It is unclear whether he has retained an attorney.

According to Sgt. David Morgenstern of the Sanford police, Zimmerman called 911 46 times since 2001.