A Chicago police officer's fatal shooting of an unarmed man -- the cop's third shooting this year -- is under FBI investigation, according to a published report.

It's a case that the chief has called "a big problem" and one that has raised questions about how authorities handle officer-involved shootings.

Gildardo Sierra, a nine-year department veteran, fired 16 shots at Flint Farmer, 29, an unemployed store clerk, in June. Farmer was hit seven times, including three times in the back, the Chicago Tribune reported Sunday (http://bit.ly/r5qxqH). The Cook County medical examiner's office said the shots to the back were fatal.

Sierra told investigators he thought Farmer was holding a gun when he spotted him on the city's South Side, but the Tribune reported Farmer had only a cell phone. The incident was captured on video from the police vehicle camera. The dark footage appears to show Farmer lying on the grass as three gunshots flash.

Police Superintendent Garry McCarthy told the Tribune that the officer shouldn't have been on the street with his history of shootings.

"He shouldn't have been where he was," said McCarthy, who took over the department in May. "We should've had him off the street so that he was not in that particular environment and that problematic type of scenario."

Sierra has been involved in three shootings since January, including two fatalities. The department ruled all three shootings justified. All three took place after midnight in two high-crime neighborhoods on the city's South Side where three officers have been killed in the last ten years.

Sierra, 31, said the video in the Farmer shooting doesn't show the entire story.

"The video does not catch everything," Sierra told the Tribune. "It's only part of the incident."

Sierra, who declined to comment further, has been stripped of his police powers. He received a 2005 distinguished service award.

In January, he and his partner opened fire during a traffic stop of an alleged gang member who was suspected in earlier gunfire. According to police reports, the driver refused to get out of his car and began dragging one of the officers. The driver, who was shot in the head, died. A handgun was later found in the car.

In March, Sierra and his partner responded to a call of shots fired. Police say a 19-year-old man pointed a gun at the officers, forcing them to shoot. The teen was hit in the leg and later charged with aggravated assault and aggravated unlawful use of a weapon. Police said they found a gun at the scene.

McCarthy said the department might have taken Sierra off the street after that shooting, but there was no system in place to track the frequency of an officer's shootings. The department has since implemented a policy in which an officer's history -- including previous shootings and mental state -- are reviewed.

"We failed as management in that particular case because there was no mechanism to review how many shootings someone was in and what their assignment is and how they were feeling," he said. "Get them to the doctor. Get them to sit down, relax, take it easy. Maybe that's an officer we want on a desk job."

The shootings are under review by the Independent Police Review Authority, but experts say the Sierra shootings raise questions.

University of Chicago law professor Craig Futterman said the department historically hasn't tracked patterns of officer behavior.

"Do they have the capacity to track patterns? Yes. Do they proactively do it? I've seen no evidence of it," Futterman said.

City officials say there have been 51 police-involved shootings in the first nine months of 2011 which already exceeds the 44 in all of last year.

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Information from: Chicago Tribune, http://www.chicagotribune.com

AP-WF-10-23-11 1845GMT