After dropped charges of murder and aggravated assault and a six-month investigation, federal prosecutors announced Thursday that they will not pursue criminal charges in the death of a man outside a D.C. nightclub in October.

U.S. Attorney Ronald C. Machen Jr. said in a statement that there was insufficient evidence to press charges. Prosecutors had interviewed witnesses and medical professionals, reviewed physical evidence and retained a “nationally renowned forensic pathologist” to help in the investigation.

Ali Ahmed Mohammed died Oct. 15 after being found on the ground outside the popular DC9 nightclub. Mohammed, 27, of Silver Spring, had thrown bricks through the window of the club after he was denied admission. Police said witnesses told them soon after Mohammed’s death that five men, all club employees, chased him, held him down, and punched and kicked him.

But prosecutors and police, who issued a separate statement Thursday, said witness statements did not match forensic evidence to support claims that Mohammed was beaten.

“The Metropolitan Police Department made arrests based on those initial reports and all of the information that was available at the time,” D.C. police spokeswoman Gwendolyn Crump said in a statement.

“The medical examiner’s office ruled the case a homicide, and indicated that the restraint of Mr. Mohammed was one of the contributing factors in his death,” Crump said. “The original accounts by witnesses were not discovered to be mistaken or inaccurate until a more comprehensive review of the medical and forensic evidence was done by detectives and prosecutors.”

In a statement released Thursday through their attorney Billy Martin, Mohammed’s family said they were disappointed that no one will be criminally prosecuted. The statement, however, indicated that the family plans to file a civil suit alleging wrongful death.

Machen said in a telephone interview that he sympathized with Mohammed’s family over their loss.

The family members “were incredibly gracious, incredibly patient. It’s obviously a tragic situation. If something similar happened to you, you would expect justice,” he said.

Machen added that he assigned three prosecutors to the investigation and that witness statements varied and some witnesses recanted parts of their statements.

“It’s our job to go where the evidence takes us,” he said. “At the end of the day, we couldn’t find any evidence to prove they were criminally responsible for the death of Mr. Mohammed. ”

Mohammed, an Ethio­pian immigrant, has been described by family members and acquaintances as a funny, friendly person, who only drank socially and had worked in recent years as a security guard and sandwich-maker in a deli.

In a news conference after his death, D.C. Police Chief Cathy L. Lanier sparked controversy by saying Mohammed was a victim of a “savage” beating and “vigilante justice” — claims that defense attorneys vehemently denied.

On Oct. 15, authorities charged the bar’s then-co-owner, William Spieler, 46, and four employees — Darryl Carter Jr., 20, Reginald Phillips, 22, and Evan Preller, 28, all of the District; and Arthur Zoloca, 25, of Silver Spring — with second-degree murder, though the D.C. medical examiner’s office had not ruled on the cause of death. A day later, those charges were reduced to aggravated assault.

On Nov. 5, prosecutors dropped those charges altogether, saying the evidence did not support their case.

The death and the subsequent dropping of charges ignited outrage and protests within the Ethiopian community around the U Street corridor, where DC9 is located on Ninth Street NW.

D.C. Council member Jim Graham (D-Ward 1), who represents the area, said Lanier’s initial comments set the tone for an agitated response in the community.

“A big question is what motivated Chief Lanier to stir us all up by terming this a savage beating,” he said Thursday night in a telephone interview. “How does she reconcile that with [the closing of the criminal case]? She set the tone for all the agitation, and now to have the final evidence that there’s no evidence for prosecution really reflects back on those earlier comments.”

The D.C. medical examiner ruled Jan. 6 that the death was a homicide. The cause of death was listed as excited delirium, associated with heart defects, alcohol intoxication and physical exertion with restraint.

On Thursday, the U.S. attorney’s statement said the manner of death was listed “forensically as homicide, which means that others were involved, though not necessarily criminally responsible.”

Prosecutors and police said in their statements that they met with the Mohammed family Thursday but did not provide details of the meeting.

Danny Onorato, a defense attorney for Preller, declined to comment when reached by telephone. Phone messages and e-mails seeking comment from Lanier and a relative of Mohammed were not immediately returned Thursday night.