A question some in Chicago are asking after the release of a video that shows a police officer fatally shooting a black teen: Did prosecutors charge the officer who killed Laquan McDonald only because they had to — because the video was about to come out?

Cook County State's Attorney Anita Alvarez rejected that notion Tuesday.

"Pressure? This is no pressure! Why — I would never be pressured into making any kind of decision, quickly," she said.

But across the country, prosecutors do seem to be under more pressure to charge police — especially in the year since police killed Michael Brown in Ferguson, Mo.

Homicide charges against police are pretty rare; they average about five cases a year. That number comes from Phil Stinson, a former-cop-turned-academic who collects statistics like this at Bowling Green State University in Ohio.

Stinson says the average is actually slightly less than five cases a year — but that's the average for the past decade. This year is looking a little different.

"As of today, we now have 15 officers who've been charged with murder or manslaughter resulting from an on-duty shooting where they've shot and killed somebody," he says.

It's an interesting jump — but Stinson's not ready to draw any conclusions yet.

"It's hard to say if we're seeing a pattern, a change in prosecutorial behavior, anything like that, because we're dealing with such small numbers," Stinson says. "We're dealing with outliers."

Statistics aside, though, he does think the justice system is giving police less benefit of the doubt than it did when he was a young cop.

"That's being chipped away," he says. "I think that now we're not taking officers at their word, and that people are looking a little bit closer. And I think that goes for prosecutors as well."

Still, there's a lot of skepticism about whether prosecutors can be objective about the police, whom they work with every day.

That skepticism grows when the decision to charge them seems to drag out, as it did in McDonald's case in Chicago, until the video came out — or in Cleveland, where it's been a year since a police officer shot 12-year-old Tamir Rice.

Jonathan Abady is one of the lawyers representing Rice's mother; he believes prosecutors there have been using that time to weaken their own case against the officer.

"It seems to us that it's taking a year because this prosecutor is more interested in protecting the police, and what they've been doing for that year is searching for people who would be willing to call what is clearly in our view an unreasonable police shooting justified," Abady says.

The prosecutor in Cleveland calls that theory "baseless," and in fact, legal experts say it really isn't fair to assume that the fix is in, just because a charging decision is taking a long time.

"It's not a race," says Laurie Levenson, a law professor at Loyola Law School and a former federal prosecutor in Los Angeles.

"There are good strategic reasons for a prosecutor actually not to bring the charges just because they can bring the charges so quickly," she says.

Once you've file charges, Levenson says, it gets harder to collect evidence against an officer.

And people don't realize how hard it is to make a case against cops; they usually have great lawyers, and they still get more sympathy from juries than the average murder defendant. Prosecutors have their work cut out for them, she says, even when there is a video.

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