The part-time police officer on a call with Cold Spring officer Thomas Decker when he was killed may have "froze" instead of going after the killer and hasn't been able to recount important details of the shooting, according to a source with direct knowledge of the case.

The source said Thursday that the team of investigators trying to find the killer is "completely befuddled" over the second officer's apparent inability to give a detailed description of Decker's killer or events surrounding the crime.

"They are looking at the other cop's actions and whether there was a failure to act," the source said.

The information is the first possible explanation of factors that have stymied investigators in the Nov. 29 killing behind a Cold Spring bar. After it was reported over police radio that an officer was down, another officer on the scene added, "I have no idea where the suspect went."

Authorities have declined to identify the second officer and have refused to answer questions about that officer's whereabouts or actions during and after the shooting. The source who spoke to the Star Tribune said that the officer's failure to provide crucial information has been a "critical roadblock" to solving the case.

"There are obvious pieces of information missing, and they are trying to determine whether this cop froze," the source said. "... They don't know why the officer can't recount what he saw."

Readying new river search

Also Thursday, authorities prepared to do another search of the Sauk River in downtown Cold Spring for the 20-gauge shotgun they believe the killer used to shoot Decker twice in the head. The source said authorities at the scene found a spent shotgun shell, a type that had contained a solid lead slug rather than shotgun pellets.

Authorities planned this time to use underwater scanning equipment in the search of the shallow river, which is just four blocks from the scene of the shooting.

Drew Evans, assistant superintendent of the state Bureau of Criminal Apprehension, which is heading up the investigation, declined Thursday night to discuss the investigation.

"We're just not in a position where we can comment right now because it's part of the active and ongoing investigation," Evans said. Stearns County Sheriff John Sanner also declined to comment. Cold Spring Police Chief Phil Jones did not return a phone message.

Decker, 31, and the second officer were checking on the welfare of Ryan Larson, 34, who lived above Winners Sports Bar and whose family was worried he might be suicidal. After a fruitless search for the killer in downtown Cold Spring that night, police found Larson in his apartment and arrested him in connection to the killing. He was jailed for several days and then released after prosecutors and police agreed they didn't have enough evidence to charge him.

Larson has asserted his innocence several times to various news organizations, saying he was asleep when the shooting happened and didn't own a 20-gauge shotgun, though he had a loaded pistol, for which he has a carry permit, and several other guns in his apartment.

Finding the gun would allow authorities to begin an ownership trace of the weapon and try to determine who last had it. Despite Larson's denials that he was involved in the killing, he "remains the primary suspect," the Star Tribune's source said. However, during the time he was held in jail, authorities also turned their attention to a Cold Spring man who has gang affiliations in Minneapolis.

Last week, investigators, citing evidence of drug dealing, executed a search warrant at a south Minneapolis residence that the second suspect was known to frequent. A man was questioned and released.

Relying on part-timers

According to the Cold Spring Police Department website, the department had seven full-time officers when Decker was shot and nine part-time officers, who under state law must be more closely supervised than full-time officers.

Part-time officers must be licensed under rules that require competency in an array of law enforcement disciplines, including use of force, search and seizures and arrests, according to the Minnesota Peace Officer training manual.

However, part-time officers cannot work without supervision of a full-time officer, who must be available by radio or have other means of direct contact with the part-timer, the manual says. Also, agencies that use part-time officers must have a policy on supervising them.

loakes@startribune.com • 612-673-1751 pmcenroe@startribune.com 612-673-1745