The U.S. Department of Justice on Tuesday cited "insufficient evidence" in its decision not to pursue civil rights charges against officers in the Freddie Gray case.

Six officers were indicted on state charges in Gray's 2015 death from injuries suffered while in police custody. Two were acquitted in bench trials. Four saw their charges dismissed. Gray's death sparked days of protest and unrest, including a riot.

Federal prosecutors reviewed the events surrounding Gray's death, including evidence uncovered by city police and local prosecutors, as well as witness interviews, Gray's autopsy and police policy documents.

To bring charges, prosecutors would have to establish beyond a reasonable doubt that an officer acted willfully to deprive Gray of a constitutional right, a standard that goes beyond the standard of negligence, accident or bad judgment.

"Although Gray’s death is undeniably tragic, the evidence in this case is insufficient to meet these substantial evidentiary requirements," the DOJ said in a statement. "In light of this... this matter is not a prosecutable violation of the federal civil rights statutes."

Officer Caesar Goodson, who faced the harshest state charges, was alleged to have given Gray a "rough ride." To pursue federal charges, the government would have to prove that Goodson intentionally and maliciously set out to hurt Gray. The DOJ said video and other evidence doesn't conclusively prove that, and noted testimony at Goodson's trial to that effect. Prosecutors also couldn't find conclusive evidence that the officers were "deliberately indifferent" to Gray's need for medical care.

The Gray family's attorney, Billy Murphy, said the Justice Department informed him on Tuesday that no federal charges would be filed. The decision means none of the officers will be held criminally responsible for Gray's death.

Five officers face internal disciplinary hearings scheduled to begin Oct. 30. Those officers are Lt. Brian Rice, Sgt. Alicia White and officers Caesar Goodson, Edward Nero and Garrett Miller. The sixth officer, William Porter, was not charged administratively.

The Justice Department decision was first reported by The Baltimore Sun. The agency, along with the Baltimore Police Department, Mayor Catherine Pugh, State's Attorney Marilyn Mosby and the U.S. Attorney's Office declined to comment on the decision.

U.S. Sens. Ben Cardin and Chris Van Hollen and Reps. Elijah Cummings, C.A. Dutch Ruppersberger and John Sarbanes, all of whom represent all or part of Baltimore, said in a joint statement that they were "disappointed" but "not surprised" and said the decision shouldn't derail efforts to reform BPD.

"In order to improve BPD and reduce the violence in our city, Baltimore will need the support of the Trump administration and DOJ," they said. "Unfortunately, the Trump administration has repeatedly stood in the way of our city’s progress by attempting to impede the implementation of the consent decree and holding hostage federal resources to reduce violent crime."

Gray's death triggered the firing of then-police commissioner Anthony Batts and prompted the Justice Department to open an investigation into allegations of discriminatory policing practices and unlawful arrests. Last year, the Justice Department released a report detailing widespread patterns of abuse and misconduct within the Baltimore Police Department, and in January entered into a court-enforceable agreement to reform the troubled agency.

On Tuesday, attorneys representing the officers expressed relief that their clients will not be held criminally responsible for Gray's death.

"These cases were never criminal and should never have been charged as such," said Rice's attorney, Michael Belsky.

Joe Murtha, who represents Porter, said he was relieved the department "determined that there wasn't a basis to move forward with the civil rights action," adding: "It's a good decision."

Michael E. Davis, an attorney for the Fraternal Order of Police Lodge No. 3, said in a statement he hadn't heard anything from the Department of Justice, but said they are "pleased" to hear media reports.

"At no time did we ever believe that there was evidence that any of the officers violated anyone's civil rights or were guilty of violating any federal laws," Davis said. "We only with that the Baltimore City State's Attorney's Office would have taken the same amount of time to conduct a more thorough investigation and if they did, we believe they would have come to the same conclusions regarding state law."