Officer Tom Decker was responding to a report of a suicidal man when he was shot twice after getting out of his squad car near an apartment behind a downtown bar. He died at the scene.
Ryan Michael Larson, 34, of Cold Spring, was held Friday on suspicion of second-degree murder. The county attorney's office was considering criminal charges.
Stearns County Sheriff John Sanner said his department got a call about 9 p.m. from Larson's family members that he might be suicidal. Cold Spring police went to his home once and couldn't raise anyone, then returned almost two hours later.
It was on the second trip that Decker was shot. He was wearing a bulletproof vest.
"It's apparent to us the officer was ambushed at the scene," Drew Evans, assistant superintendent of the state Bureau of Criminal Apprehension, said.
Police with dogs searched Friday for a gun near the site of the shooting in downtown Cold Spring, which is about 20 miles southwest of St. Cloud. Brian Moen, who lives about a block from the bar, said officers who came to his door told him they were looking for a sawed-off shotgun.
Cold Springs Police Chief Phil Jones described Decker as a "chief's dream." Jones said Decker grew up on a farm south of town, and after graduating from college, worked at several small Minnesota police departments before coming home for what he called his dream job.
"He was a hometown boy," Jones said.
He described Decker as the "department jokester" on a force with only eight full-time officers. Decker served as the department's instructor on firearms and use of force.
"Not only did I have no problems with him, but he was the type of officer who accumulated six letters of appreciation and commendation in six years with us," Jones said. "We lost an officer, the community lost a citizen."
Decker leaves behind a wife and four young children -- two sons and two daughters, his chief and family members said.
Joe Decker, his younger brother, told The Associated Press that Tom Decker loved to travel and be outdoors. Joe Decker said his brother was shy and reserved as a youngster but became outgoing and gregarious as an adult.
"He was one of those people who'd be the life of the party," Joe Decker said.
The Deckers' mother, Rosella Decker, told the Star Tribune that the family knew Tom's profession was dangerous.
"He had a lot of little close calls, and he would tell me about them afterward," she told the newspaper.
Larson had drawn police attention before in the community of about 4,000, mostly for traffic-related offenses but once in an abuse case.
In 2009, he reached a plea agreement to settle a misdemeanor disorderly conduct charge for engaging in behavior that could "arouse alarm, anger, or resentment in others." As part of the plea deal, he served a day in jail and had another three months stayed, but he agreed to undergo domestic abuse counseling. A no-contact order was issued. Court files indicated he violated his probation in 2010.
Civil court records show Larson was sued more than once over outstanding debt and was evicted from a rental property in 2008 for failing to pay his rent.
Larson's relatives either couldn't be reached or declined to comment. One said she wasn't sure whether Larson had an attorney.
Condolences came from fellow police officers in other Minnesota departments and from elected officials, including Gov. Mark Dayton.
"Cold Springs Police Officer Tom Decker was senselessly murdered last night, while acting in the line of duty," the governor said. "On behalf of the people of Minnesota, I extend my deepest sympathies to his family and to the Cold Springs Police Department for their tragic loss of an outstanding officer, father and friend. Officer Decker died, while protecting his fellow citizens. For his heroism, we will be forever grateful."