More than 100 protesters staged a sit-in at a gas station in St. Louis near the site of a police-involved shooting last week, prompting riot police to deploy tear gas and make arrests.

More than 100 protesters staged a sit-in at a gas station in St. Louis near the site of a police-involved shooting last week, prompting riot police to deploy tear gas and make arrests.

Justice Department investigators have all but concluded they do not have a strong enough case to bring civil rights charges against Darren Wilson, the white police officer who shot and killed an unarmed black teenager in Ferguson, Mo., law enforcement officials said.

When racial tension boiled over in Ferguson after the Aug. 9 shooting, Attorney General Eric H. Holder Jr. traveled to the St. Louis suburb to meet with city leaders and protest organizers in an effort to bring calm. He assured them that the federal government would open a civil rights investigation into the fatal shooting of Michael Brown. But that investigation now seems unlikely to result in any charges.

“The evidence at this point does not support civil rights charges against Officer Wilson,” said one person briefed on the investigation, who spoke on the condition of anonymity because of the sensitivity of the case.

Justice Department officials are loath to acknowledge publicly that their case cannot now meet the high legal threshold for a successful civil rights prosecution. The timing is sensitive: Tensions are high in greater St. Louis as people await the results of a grand jury’s review of the case.

Many supporters of Brown say they are already convinced there will be no state-level indictment of the officer. Federal officials have wanted to show that they are conducting a full and fair review of the case.

Ferguson Police Chief Thomas Jackson and Missouri State Highway Patrol Capt. Ronald Johnson held news conferences Friday to discuss the release of robbery surveillance video, reveal the identity of the officer who shot Michael Brown and more. (Nicki DeMarco/The Washington Post)

Justice spokesman Brian Fallon said the case remains open and any discussion of its results is premature. “This is an irresponsible report by The Washington Post that is based on idle speculation,” Fallon said in a statement.

Other law enforcement officials interviewed by The Post said it was not too soon to say how the investigation would end. “The evidence we have makes federal civil rights charges unlikely,” one said.

A lawyer for Brown’s family, Benjamin L. Crump, said he would not comment “on something that is not official.”

James P. Towey Jr., Wilson’s attorney, did not return calls or e-mails seeking comment.

The Justice Department is continuing its broad investigation of the policing practices of the Ferguson Police Department, which could result in wholesale reforms and reorganization. The Justice Department on Friday announced an agreement with the city of Albuquerque intended to overhaul the way its police department uses force, the result of one such civil rights investigation.

At a forum this week organized by the Aspen Institute and the Atlantic magazine, Holder indicated that a similar overhaul could be called for in Ferguson. “It’s pretty clear that the need for wholesale change in that department is appropriate,” Holder said.

Federal law sets a high bar in bringing civil rights charges against a police officer because prosecutors must prove beyond a reasonable doubt that the officer intended to violate someone’s constitutional rights.

Authorities faced a similar challenge in the investigation of George Zimmerman in the 2012 shooting death of unarmed black teenager Trayvon Martin in Sanford, Fla. Under federal law for hate crimes, prosecutors have to show that someone has been victimized intentionally because of a racial or other bias.

Law enforcement officials have said privately that there is insufficient evidence to bring federal charges in that case, although the two-year probe technically remains open.

The investigation of the Brown shooting is being conducted by the Justice Department’s Civil Rights Division under a federal statute that makes it a crime for a person with government authority — the legal term is “acting under color of any law” — to “willfully deprive a person of a right or privilege protected by the Constitution or laws of the United States.”

Sometimes the department is successful. In 2010, prosecutors won convictions of two New Orleans police officers for civil rights violations in connection with the killing of a man and the burning of his body during the disruption that followed Hurricane Katrina. The officers have appealed their convictions.

Holder and other officials have decried recent news reports about investigative findings in the Ferguson case that have revealed new but conflicting details about the three-minute encounter between Wilson and Brown. Some of those details potentially corroborate the officer’s account that the killing was an act of self-defense, and they could complicate a civil rights case against Wilson.

The St. Louis County autopsy report, published Oct. 21 by the St. Louis Post-Dispatch, was interpreted by some forensic pathologists as indicating that Brown may have struggled for control of Wilson’s gun during their initial altercation, but they also said the evidence was inconclusive.

After two shots were fired inside Wilson’s patrol vehicle, the officer got out and Brown fled but later turned around as Wilson continued firing. Some pathologists said the report indicates — but not conclusively — that Brown’s hands were not over his head. Several witnesses said his arms were raised in surrender when the officer shot him again.

Rachel A. Harmon, a law professor at the University of Virginia and a former prosecutor in the Justice Department’s Civil Rights Division, said it is especially challenging to prove a civil rights case beyond a reasonable doubt.

“There is an extra burden in federal civil rights cases because the statute requires that the defendant acted ‘willfully,’ ” Harmon said. “It is not enough to prove that he used too much force. You have to prove beyond a reasonable doubt that he did so willfully.”

Harmon also said that if Wilson “genuinely believed he was acting in self-defense,” then his actions are not considered “willful,” meaning he did not intend to deprive Brown of his constitutional rights.

Brown was shot a total of nine times, including three times in the head, according to the county autopsy.

Dorian Johnson, the 22-year-old who was with Brown when the two encountered Wilson, has said the officer was the aggressor and did not act in self-defense.

David Klinger, a former Los Angeles police officer and now a professor of criminology at the University of Missouri at St. Louis, said enduring disputes over what happened are likely to raise reasonable doubt that would make a successful civil rights prosecution almost impossible.

“The autopsy report is devastating because it raises doubts about him standing still with his hands in the air in surrender,” said Klinger, who fatally shot a suspect in the line of duty when he was an officer. “If you have a halfway competent lawyer, the defense could raise reasonable doubt with this.”

Samuel Bagenstos, a former Justice Department principal deputy assistant attorney general for civil rights and now a law professor at the University of Michigan, said the obstacles prosecutors face in the Ferguson case are typical, as are the frustrations of Brown’s supporters.

It is common to have a situation “that looks like a constitutional violation and may well be an injustice,” Bagenstos said. “But sometimes the Justice Department does not have the ability to bring a civil rights case under the statutes it enforces.”