Cold Spring shooting suspect released

DO NOT USE. LARSON WAS CLEARED Ryan Larson
In this photo provided by the Stearns County Sheriff is Ryan Larson, 34, of Cold Spring, Minn. Larson was arrested at the scene of a shooting Thursday, Nov. 29, 2012 in Cold Spring, Minn., where police officer Tom Decker died in what authorities are calling an ambush killing. The county prosecutor is reviewing the case for charges.
AP Photo/Stearns County Sheriff

Police have released the suspected killer in the Cold Spring shooting death of a police officer due to lack of evidence.

From MPR News:

Stearns County Attorney Janelle Kendall says there's not enough evidence to charge Ryan Larson, 34, with the murder of Cold Spring Police Officer Tom Decker.

"There is not sufficient evidence at this time to continue to hold him," Kendall told MPR News. "Mr. Larson was arrested. That was a logical choice at the time and now the active investigation is ongoing."

Police still haven't found the shotgun that they believe killed Decker Thursday night in what they describe as an ambush.

What is required to charge a suspect? What we can expect in the case going forward?