Cook County prosecutors said Tuesday that they would not bring criminal charges against a Chicago police officer who was captured on videotape fatally shooting an unarmed man, saying an exhaustive review showed that the officer reasonably mistook a cellphone for a gun pointed at him on a darkened street.

The death of Flint Farmer in June 2011 was the third shooting — the second fatal shooting — in six months by Officer Gildardo Sierra, a patrolman in the Englewood district. It was so disturbing that it prompted police Superintendent Garry McCarthy to tell the Tribune that he considered the shooting a "big problem" and to acknowledge the department had erred by allowing Sierra back on the street given the previous shootings.

In December, the city of Chicago settled a lawsuit brought by Farmer's estate for $4.1 million without admitting wrongdoing.

Still, Cook County prosecutors said their two-year investigation of the shooting showed that Sierra had reason to believe that Farmer was armed and posed a threat of "great bodily harm." They said that although Sierra fired his weapon 16 times, hitting Farmer seven times — including three times in the back — they did not think they could show that the shooting was unreasonable, a key component of proving that Sierra had committed a crime.

"Although Officer Sierra was mistaken in his belief that Flint Farmer had a gun, not every mistake demands the action of the criminal justice system, even when the results are tragic," State's Attorney Anita Alvarez said Tuesday in a letter to McCarthy.

In addition to providing the Tribune with the letter to McCarthy, four officials from Alvarez's staff sat down with reporters for three hours to explain the investigation and make a video presentation. They were: chief of staff Dan Kirk; Jack Blakey, chief of the office's special prosecutions bureau; spokeswoman Sally Daly; and Assistant State's Attorney Nick Trutenko, who conducted the bulk of the investigation. The Tribune ran a front-page story in 2011 raising questions about the shootings by Sierra.

The prosecutors said although the videotape of the shooting was damning, showing muzzle flashes and suggesting Sierra stood over Farmer as he shot him in the back, the continued investigation yielded forensic and other evidence that led the prosecutors to conclude that the incident was more complex.

"The video is actually somewhat maddening," Trutenko said. "It's why we run out every ground ball."

Farmer's father, Emmett, wept Tuesday when told by the Tribune about the prosecutor's decision. In the more than two years since the shooting, he has attended several rallies calling for Sierra's prosecution and firing. He said the settlement of the civil lawsuit was, in the end, cold comfort.

"That's just horrible," he said as his voice broke. "I've seen the video. My son is lying on the ground and Sierra fires three bullets into his back. You can't tell me it's self-defense. I don't care how quickly the whole thing happened. You can't shoot a man in the back and call it self-defense. My son didn't even have a gun."

Sierra's attorney, Matthew McQuaid, praised the decision not to seek charges, saying it was the only one to make based on the evidence.

"This was never a criminal case," McQuaid said. "He (Sierra) was adamant about what happened and that he acted in self-defense."

The shooting occurred shortly before 2 a.m. on June 7 when Sierra and a partner responded to a call of a domestic disturbance allegedly involving Farmer and his girlfriend in the 6200 block of South Honore Avenue.

"My baby daddy, he just jumped on me and he just beat up on me and my kids real bad," Farmer's girlfriend, Tanesha Whitaker, can be heard saying on a 911 call that prosecutors played.

When police arrived, Farmer fled to South Wolcott Avenue, one street west. He got as far as the parkway when Sierra reportedly yelled at him.

Prosecutors said Farmer, 29 and unemployed, pointed his burgundy cellphone at Sierra, prompting the officer to fire all 16 rounds from his handgun. A patrol car that arrived during the brief confrontation captured video of Sierra as he stepped onto the parkway, walked around the injured and unarmed Farmer in a semicircle and fired three more shots. That video, which the Tribune placed online two years ago, raised questions about whether the shooting was justified.

An autopsy by the Cook County medical examiner's office showed that the three shots in Farmer's upper back were the fatal wounds.

Before Farmer's death, Sierra had wounded a 19-year-old man in a shooting in March 2011 and killed Darius Pinex, 27, in January 2011. All three shootings took place after midnight in the impoverished and crime-ridden Englewood and West Englewood neighborhoods.

The Police Department ruled Farmer's shooting justified, but McCarthy later told the Tribune, without identifying Sierra by name, that he considered the case "a big problem" and that the officer involved should not have been on the street. Sierra later admitted that he drank "multiple" beers before he went to work that night, but the city waited more than five hours to give him a breath test, according to a court filing by an attorney for the slain man's estate.

The Independent Police Review Authority, the city agency that investigates shootings involving officers, referred all three shootings by Sierra to prosecutors for possible criminal prosecution. The FBI also had been investigating the shootings, but it is unclear where that inquiry stands. Sierra, 33, who joined the department 11 years ago, was stripped of his police powers and has been assigned to desk duty since the Farmer shooting.

Prosecutors said that by slowing down the video and syncing it with 911 audio and other information, they were able to draw a better picture of what happened — a picture that corroborated Sierra's statements to officers that he feared for his life. The prosecutors did not interview Sierra and would not say if they even sought to question him.

Prosecutors pointed to several key pieces of evidence in deciding against charging the officer. One was a wound to Farmer's right hand that suggested he was pointing his arm at Sierra when he was shot. Prosecutors believe that was one of the first shots, if not the first, to hit him. In addition, DNA tests showed that blood on Farmer's phone was his, suggesting he was holding the phone when shot. The other shots followed, with the last three hitting Farmer in the back.