ST. PAUL, Minn. — Authorities investigating last month’s killing of a central Minnesota police officer announced Monday that a reward of up to $100,000 is being offered for information that helps solve the case, and they are specifically seeking details about a black van seen in the Cold Spring area on the night of the slaying.

Cold Spring Officer Tom Decker, 31, was fatally shot as he was checking on a man reported to be suicidal. Authorities have called the killing an ambush. Police arrested a 34-year-old suspect, but released him five days later because there wasn’t enough evidence to charge him.

The reward is one of the largest ever offered in Minnesota by the nonprofit Spotlight on Crime. Authorities hope it will spur someone to come forward about a killing that has left many questions: No murder weapon has been found, and a court document used to detain the man initially arrested in the case indicates the other officer on the scene retreated and lost sight of the suspect.

“The bottom line is: This is a cold-blooded killing and the sooner that we get this individual that is responsible for this behind bars, the better all of us will sleep at night,” Stearns County Sheriff John Sanner said. “Law enforcement officers swear an oath to protect and serve. When an officer is killed discharging those duties, we are all left feeling a little bit more vulnerable.”

Investigators said Monday they have received more than 200 tips and are doing everything they can to find Decker’s killer. A team of local, state and federal agents is dedicated to the case. Bureau of Criminal Apprehension Assistant Superintendent Drew Evans said a team of divers, including some from the FBI, was searching the Sauk River on Monday for the 20-gauge shotgun believed to be the weapon used in the slaying.

Evans said authorities also are seeking information about a black van with a loud exhaust that was seen that night leaving the area where the killing occurred.

Cold Spring Police Chief Phil Jones said “Tommy” was more than just a fellow officer — he was a friend.

“Right now it feels like we all have a hole in our hearts that can’t be filled,” he said. “Finding the killer would start the healing.”

Decker, a father of four, was fatally shot Nov. 29 behind a downtown Cold Spring bar as he and Officer Greg Reiter were checking on the welfare of a man who lived above the bar. Reiter went to the scene first and then called Decker for backup, according to a document used to keep the initial suspect in jail in the days after the shooting.

Reiter stayed in his car when Decker began walking toward the man’s apartment, according to the document.

“Officer Reiter, still seated in the squad car, then heard two loud ‘bangs,'” the document said.

Reiter reported he saw a 6-foot-tall white man in a hooded sweatshirt, jeans and dark stocking cap standing in front of Decker’s squad car and pointing a handgun in a shooting position, according to the document. Reiter stayed in his squad car and drove in reverse.

“Officer Reiter then lost visual contact,” the document said.

In radio transmissions from the night of the killing, which have been archived online, a voice is heard saying: “Officer down.” Moments later, a man says: “I have no idea where the suspect went.”

There was no answer at Reiter’s home on Monday when The Associated Press tried to reach him for comment. While the document says Reiter saw a handgun, Evans said evidence at the scene indicates a shotgun was used.

Evans said no one has been ruled out as a suspect, including the man initially arrested. When asked if Reiter was also a suspect, Evans said the investigation is still active, and Reiter has been cooperative. Reiter is on administrative leave, which is standard.