Federal prosecutors will not bring criminal charges in the case of an unarmed black teenager who was shot to death in his home by a white New York City police officer, officials said Tuesday.

The U.S. attorney's office in Manhattan said prosecutors found insufficient evidence to pursue federal charges in the 2012 death of Ramarley Graham and have officially concluded their investigation. The 18-year-old was shot in the bathroom of his Bronx home by an officer who had barged inside during a drug investigation. He was killed in front of his grandmother and 6-year-old brother.

In a statement Tuesday, U.S. Attorney Preet Bharara said his office conducted a thorough and independent investigation, but determined "there is insufficient evidence to meet the high burden of proof required for a federal criminal civil rights prosecution."

Prosecutors have said police first encountered Graham when they spotted him and two other people walking into a Bronx bodega in the afternoon of Feb. 2, 2012 and then immediately walking out. The officers, who were conducting a street narcotics investigation, said they saw Graham adjusting his waistband and told fellow officers they believed he had a gun. Police followed him to his Bronx home. An officer made his way into the home and forced his way into a bathroom and shot Graham once.

Richard Haste, the officer who shot Graham, said he fired his weapon because he thought he was going to be shot. No weapons were found in the apartment.

Haste was initially indicted in the Bronx on a state manslaughter charge, but a judge dismissed the case after determining that prosecutors improperly instructed grand jurors. A new grand jury declined to re-indict the officer.

After the shooting, Haste was stripped of his badge and gun and assigned to the department's fleet services division, officials said, but an internal disciplinary proceeding against him has been on hold pending the outcome of the federal investigation. In that time, Haste has received raises guaranteed by his union contract. A spokesman for the New York Police Department said Tuesday that Haste is currently on "modified assignment."

"He's gratified that the federal government has properly determined that there were no civil rights violations," Haste's attorney, Stuart London, said. "There never were any winners in this case because there was a loss of life."

Graham's family received a $3.9 million settlement from New York City.

Last month, Graham's parents and civil rights activists held an overnight protest at Bharara's office, sleeping on the concrete steps of the Manhattan office building, protesting what they believed was a lag in the investigation. They have also repeatedly sent letters to local and federal officials calling for Haste to be fired.

Graham's father, Frank Graham, told the New York Daily News that Tuesday's decision was "heartbreaking" and "frustrating."

"But we'll just move onto the next fight — which is firing the officers immediately," he said.

The Rev. Al Sharpton, who delivered the eulogy at Graham's funeral, said the decision to not pursue federal charges against the officer was "very painful" for Graham's family and the public.

Graham's death has been cited during numerous demonstrations after grand juries in Missouri and New York declined to indict police officers in the deaths of Michael Brown in Ferguson, Missouri, and Eric Garner on Staten Island. The deaths fueled a national conversation about policing and race. A federal grand jury is currently hearing evidence in Garner's case.