December 4 2015 New York NY Protestors take to the streets in NYC from Times Square to China Town marching, laying on the ground and chanting. Matthew Nelson/MediaPunch/IPX

Over the last 15 years, NYPD officers have killed at least 179 people, according to a new investigation.

The New York Daily News found that in only three of those incidents, the officer involved was indicted and only once was the cop convicted.

In that one instance, when ex-officer Bryan Conroy was convicted in 2005 of criminally negligent homicide for killing Ousmane Zongo, Conroy didn't serve any jail time.

Patrick Lynch, head of the Patrolmen’s Benevolent Association, defended the NYPD officer's actions.

"When there is a life-or-death situation on the street, be it an armed robbery, a homicidal maniac on the street or someone driving a vehicle in a dangerous and potentially deadly way, it is New York City police officers who step in and take the risk away from the public and put it on themselves,” Lynch said in a statement. “Our work has saved tens of thousands of lives by assuming the risk and standing between New Yorkers and life-threatening danger.”

To be sure, some of the incidents catalogued by the Daily News involved the justified use of deadly force by officers.

But, holding cops accountable when they are not justified in killing someone is difficult, because often the prosecutors tasked with bringing charges against officers also rely on good relationships with police to do their day-to-day work. DA's also count on endorsements from police unions when they run for re-election.

The recent decision not to indict Daniel Pantaleo in the Eric Garner chokehold case, has set off calls for laws requiring special prosecutors in cases involving possible police misconduct.

The idea behind any proposed legislation would be to keep local district attorneys out of cases where they might be biased in favor of the police department they work with regularly.

But some, like panelists involved in a recent Democracy Now discussion, said such reforms have been sought for years and have little chance of becoming law, at least at the federal level.

Harry Siegel, a columnist for the Daily News, pointed out that New York Gov. Andrew Cuomo, who recently said special prosecutors could be necessary in some cases, had the chance to appoint a special prosecutor in the Garner, case but didn't.