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The sex crime investigation began with the clue of a single letter: C.

Minneapolis Police Sgt. Bernie Martinson seized on the tiny tip in an otherwise unremarkable note written by an inmate named Jon R. Moran. By the time Martinson was done, he had found 22 sex assault victims and Moran was headed for some serious time.

Martinson's former supervisor, Lt. Nancy Dunlap, used the case as a prime example of the determined cop's work when she handed him a career-achievement award at a recent Minnesota Sex Crimes Investigators Association meeting.

To the public, Martinson is largely unknown. In the Minneapolis Police Department, he is near legendary. He slipped into retirement late last month without notice by anyone but his colleagues and his boss, who dreaded the departure of the indefatigable investigator.

The "C" case began when Martinson got a call from the state Department of Corrections about a note Moran wrote to a fellow inmate about his time on the run from treatment. Moran wrote that he had "offended against a certain young female named C. Other than that snafu, I am doing fine."

Martinson knew Moran from a 2003 case. He asked his boss for permission to investigate. "I'm not sure it involves Minneapolis, but I'd like to work on it and I won't let it take away from my other cases." Dunlap said she told him to "go for it." She knew that was how Martinson generated many of his cases.

From Rush City to Hastings and Mankato, Martinson interviewed Moran, his fellow inmates and therapists. He learned of 22 possible victims, including two who alleged 70 assaults.

Moran pleaded guilty in Dakota County District Court to charges that grew from the investigation. He also was recently indicted in Hennepin County on additional charges. If convicted, he could be held for a lifetime civil commitment -- all because of Martinson's initiative.

Martinson, 53, was always a workhorse. He carried the heaviest caseload and had the highest charging rate in the unit, Dunlap said.

In his 24-year career, he personally investigated and steered to civil commitment 100 mentally ill and dangerous sexual psychopaths.

He trained other investigators, served on state panels and became a computer forensic expert on child pornography. At the office, he was a practical joker skilled in the delivery of punch lines. He would put up office holiday lights and decorate a tree with ornaments of curious design.

His colleagues called him the "Energizer Bunny," but he ran on popcorn, cookies and Diet Coke. Martinson had no time for meals, his boss said.

Dunlap's award speech concluded with a thank you letter to the detective written by a frightened victim who praised his kindness. "I have begun to put my life back together; returned to college and am working two jobs. I don't think any of this would have been possible without your professional and compassionate handling of my case," she wrote.

Dunlap said Martinson summed up his career in a typically sly single sentence: "24 years of bad sex."

Rochelle Olson • 612-673-1747 @rochelleolson