A special prosecutor raised questions about a Jefferson County commissioner's relationship with a private investigator - and found other behavior "very concerning" and "inexplicable" - but will not file criminal charges against him.

Thursday's decision by Adams County District Attorney Don Quick came after a yearlong investigation into charges that Jefferson County Commissioner Jim Congrove improperly retained a private investigator to conduct surveillance on a longtime critic of the county.

But while Quick found no grounds to file criminal charges, he used words like "troubling" to describe some of the things he found.

"Whether the conduct described herein is a 'best practice' or 'appropriate policy' for county officials supported by county taxpayers is not a question for me," Quick wrote. "The answer to that question appropriately vests with Jefferson County government and the citizens of Jefferson County."

Reached at a meeting Thursday afternoon, Congrove said he had not seen the decision and said he would comment later. But he did not answer multiple calls or return a message left by the Rocky Mountain News.

The investigation was the result of a complaint by a watchdog group that questioned whether Congrove had inappropriately spent county money to hire private investigator Daril Cinquanta, a longtime friend.

Cinquanta conducted surveillance on the critic, Mike Zinna, as well as on Zinna's attorney and a business associate, and filmed people who made comments at several meetings of the commissioners.

Records show that Jefferson County paid Cinquanta's firm $7,462 in 2005 and 2006.

Quick was appointed special prosecutor in the case after Jefferson County District Attorney Scott Storey recused himself.

Much of the surveillance was aimed at Zinna, who in addition to his frequent criticism of county government filed a lawsuit against the county alleging that officials breached a contract for development of land at the airport near Broomfield.

Quick concluded that he could not prove that Cinquanta's work did not benefit the county. Therefore, he said, he could not file charges.

Chantell Taylor of Colorado Ethics Watch, who filed the complaint, said she was "disappointed" but "not exactly surprised" by Quick's decision.

"District Attorney Quick makes it pretty clear that he has concerns about Commissioner Congrove's conduct, but it just didn't rise to the level of criminal misconduct," Taylor said.

Cinquanta, a former Denver police officer, had not seen Quick's decision in what he termed "a yearlong inquiry that I think was very unfair."

"What a waste of taxpayers' money," he said.

Zinna said the district attorney made the right decision because the scope of the investigation was limited.

"I don't think anybody believed it was criminal to hire (Cinquanta)," Zinna said. "I think what people found reprehensible is what Daril Cinquanta did after he was hired."

Zinna said he thinks an ongoing investigation by the Jefferson County Sheriff's Office into missing files, which authorities said include details of Cinquanta's work, could lead to criminal charges.

Jefferson County Commissioner Kathy Hartman, a Democrat elected in November 2006, said she respected Quick's decision not to proceed with a trial at taxpayer expense if it was unlikely to result in a conviction.

Hartman replaced Republican Dave Auburn, who has not been accused of anything. But she said her election was due in part to voter distrust of the commission and added that the investigation has done nothing to help that image.

"I do think it taints all of us," Hartman said.

Asked if she thinks Congrove should step down, Hartman said: "That's for voters to decide."

Auburn also said he was not surprised.

"I don't think it was criminal," he said. "I do think it was a misuse of funds."

Quick's decision may not be the final word.

The sheriff's office continues to investigate files in the case that disappeared from the office of Assistant County Attorney Duncan Bradley. And documents that had been in a rolling file in Congrove's office also are missing.

In his letter announcing the decision, Quick said it was "very concerning" that when Congrove was questioned by investigators about the rolling file, he did not tell them "that the rolling cabinet was now empty and in his office."

Quick said that when he considered the fact that Congrove was a former police officer, he found the omission "inexplicable."

Finally, Quick raised questions about Congrove's plan to contribute $33,000 to a nonprofit corporation affiliated with Cinquanta.

The money was part of a $100,000 grant from the Kaiser-Hill Legacy Foundation to the commissioners - and each commissioner was allowed to distribute one-third of the money to nonprofits.

Congrove sought to give his share of the grant to Rocky Mountain Aviation, which listed Cinquanta as its registered agent. Someone in the county's budget office raised questions, however, and according to Quick's report, the money will not be given to Cinquanta's group.

Quick's letter noted that Congrove attempted to make the contribution while he was under investigation.

Correspondent Marilyn Robinson contributed to this report.