MINNEAPOLIS (WCCO) – It is highly unusual for a person accused of killing a police officer to be arrested and then released without charges because there isn’t enough evidence.

According to the Officer Down Memorial Page, over the past 20 years, 13 Minnesota law enforcement officers have been killed on duty. In six of those cases, the killer was shot at the scene. Six others went to court.

But what evidence does it take to charge someone with murder?

“You just have a feeling maybe the right person was arrested. But having a feeling, having a suspicion, that isn’t enough,” said former Ramsey County Attorney Susan Gaertner, now in private practice at Gray, Plant, Mooty.

Gaertner personally prosecuted the killer of St. Paul Police Officer Jerry Vick.

“This was, I’m sure, a very hard situation to deal with,” she said, regarding the Cold Spring incident. But on the law, however, it wasn’t tough.

“My understanding is the prosecutor said it wasn’t a close call,” Gaertner said.

Typically, prosecutors get 48 hours from when someone’s arrested to take them to court. That gets extended over a weekend or holiday. Then, they need to decide.

“You can successfully prosecute murder without a weapon, without a confession, without a solid eyewitness. But you can’t do it with none of those things,” Gaertner said.

You can’t do it with only one of those things either. Sometimes people confess to things they didn’t do. Sometimes the ballistics aren’t as clear as they seem. Sometimes eyewitnesses are wrong.

“The point is: There’s no checklist,” Gaertner said.

After a prosecutor files charges, a judge almost never throws them out, because a prosecutor only needs to prove probable cause. But ethically, according to Gaertner, there’s a higher standard.

“Unless you believe that ultimately when you’re in front of a jury you can prove the case beyond a reasonable doubt,” you don’t, she said.