(Gillian Brockell/The Washington Post)

Acting U.S. attorney Corey Amundson announced Wednesday that the Justice Department will not bring charges against the police officers involved in the fatal shooting of Alton Sterling, asserting that all the federal prosecutors and agents who investigated the matter found “insufficient evidence” to proceed with a case.

The announcement confirmed officially what The Washington Post reported Tuesday, although Amundson and the Justice Department revealed far more detail about how investigators came to the conclusion they did. Amundson, the acting U.S. attorney for the district that includes Baton Rouge, laid out a thorough timeline of the encounter — which was captured on a video that had rocketed around social medial last summer — but said one major unanswered question proved decisive in the case.

Investigators were not able to determine whether Sterling, 37, was reaching for a gun when an officer yelled that he was, Amundson said, and they thus could not prove that officers knew what they were doing was unreasonable when shots were fired.

“There are no winners here, and there are no victories for anybody,” Amundson said. “A man has died, a father, a nephew has died. My heart goes out to the family.”

The lawyers and family of Alton Sterling at a press conference on Wednesday. (Bryan Tarnowski for The Washington Post)

The case is not completely over. Louisiana Attorney General Jeff Landry said that while he had been precluded from investigating the matter while the Justice Department did its work, he would now launch a state probe. He said he had asked the Justice Department to forward its investigative materials to Louisiana State Police and assigned a prosecutor with the Louisiana Department of Justice to the case.

Landry said his investigation “could take a considerable amount of time.”

Attorneys and Sterling’s family said they hope state officials bring charges and that they left a meeting with federal investigators Wednesday feeling more enraged than they had before.

Chris Stewart, the lead attorney for the Sterling family, said that investigators told family members that at the beginning of the interaction with Sterling, Officer Blane Salamoni put his gun to Sterling’s head, and said, “I’ll kill you, b----.”

That statement was not included in the Justice Department’s public description of the encounter; the Sterling family attorneys said it was gathered from not-yet-released audio of the interaction. Sandra Sterling, an aunt who raised the young man after his mother died, said it was particularly jarring.

“Salamoni put that gun to his head and said, ‘I’m going to kill you,’ ” she told reporters Wednesday. “How do you think Alton felt? How do you think Alton felt after hearing that?”

Cameron Sterling, 15, said that he forgives the officer who killed his father, but he is hopeful that state prosecutors bring charges.

Sandra Sterling, left, and Cameron Sterling listen as the lawyers and family of Alton Sterling hold a press conference on Wednesday. (Bryan Tarnowski/Bryan Tarnowski for The Washington Post)

“I just hope that justice will be served for my dad,” Sterling said. “Because if not, it will be a repeated cycle, where they will feel like they can do something like this again.”

An attorney for Salamoni declined to discuss the allegation that the officer threatened Sterling before the shooting, and he said he thinks state investigators will likely also decline to press charges.

“Because there is a pending state investigation, I can’t comment on the evidence,” said John McLindon, the Baton Rouge attorney for Salamoni. “The federal government has seen every piece of evidence. And now I think the state attorney general will do a very thorough and exhaustive investigation, and I think the result will be the same.”

Fred T. Crifasi, an attorney for Howie Lake, the other officer who responded, said that his client was “relieved,” but he declined to comment further.

Substantiating federal civil rights charges is a difficult endeavor, requiring prosecutors to produce evidence that demonstrates an officer’s intent at the time of an incident. Amundson said that it would not be enough to show that the officers who encountered Sterling acted recklessly, negligently or even that they escalated a situation they should have de-escalated.

The Justice Department offered this description of the encounter:

At about 12:30 a.m. on July 5, someone called 911 to report that a black man wearing a red shirt and selling CDs had “pulled a pistol” and had a gun in his pocket. Salamoni and Lake responded and saw Sterling, dressed in a red shirt and standing by a table with a stack of CDs.

The encounter lasted less than 90 seconds from that moment until the firing of the final shot. Officers told Sterling to put his hands on the hood of a car, and when he did not, a struggle ensued. Salamoni pulled out his gun and pointed it at Sterling’s head, and Sterling put his hands on the hood. When Sterling moved his hands, Lake shot him with a Taser.

A scuffle ensued, and Salamoni tackled Sterling. Lake knelt on Sterling’s left arm, while Salamoni tried to get control of his right. Salamoni then yelled: “Going for his pocket. He’s got a gun! Gun!”

“Less than one second later, during a point at which the location of Sterling’s right hand was not visible to the cameras, Officer Salamoni again yelled that Sterling was ‘going for the gun!’ ” the Justice Department said in a news release. “Officer Salamoni then fired three shots into Sterling’s chest.”

That detail, Amundson said, was critical. To substantiate civil rights charges, he said, prosecutors would “have to prove, beyond a reasonable doubt, that Officer Salamoni did not believe that Mr. Sterling was actually going for a gun when he made that statement.”

The Justice Department said that after the first three shots were fired, Sterling, whose back was to the officers, began to sit up and roll to his left, bringing his right arm across his body toward the ground. Lake yelled for him to “get on the ground,” and as he continued to move, Salamoni fired three more shots into his back, the Justice Department said.

The Justice Department said the officers recovered a loaded, .38-caliber revolver from his pocket, though that detail itself was not especially revelatory. It was not in dispute that Sterling had a gun, but whether he reached for it, as the officers claimed.

At a news conference Wednesday, Baton Rouge Mayor-President Sharon Weston Broome (D) urged people to keep calm and said the Justice Department’s decision not to charge “does not mean the police officers acted appropriately.”

So far, the protests in response to the Justice Department’s decision have been more subdued than those held in July after Sterling’s killing by police.

Sgt. L’Jean McKneely of the Baton Rogue Police Department said police also have been conducting an internal investigation into the shooting. The officers involved — Salamoni and Lake — are on paid administrative leave and will remain that way at least through the conclusion of the state attorney general’s probe, McKneely said.

Zapotosky and Lowery reported from Washington.