When does an unsolved crime become a cold case, no longer worth investigation? Twenty years? Fifty years? A century?

None of the above, according to Tom McLellan, a 32-year veteran of the Fort Collins police department, and the author of a fascinating new book, “FCPD: A History of the Fort Collins, Colorado Police Department 1873-2013.”

“We can’t give up,” says McLellan. “In 2005, I came across brand-new evidence 98 years after one of the worst crimes in our community, the murder of a Fort Collins police officer.”

Strolling today down placid Linden Street toward the river, one would struggle to imagine that part of Old Town in the early 1900s. Avoided by most local residents, “The Jungles” was a lawless den of gamblers, moonshiners, bootleggers and prostitutes. The authorities turned a blind eye, satisfied to let the wound fester because — according to prevailing gossip — a few noteworthy politicians also happened to be The Jungles’ clients.

But one cop took it upon himself to clean up the dangerous neighborhood. Joseph N. Allen was a 45-year-old husband and father of two who struggled to support his family in their hometown of Parsons, Kansas. So he rode his bicycle to Northern Colorado to find gainful employment, hiring on with the city of Fort Collins as a night policeman. He was only two months on the job on the night of July 3, 1907, when he told his landlady before reporting for duty, “Well, you may not see me again until after the Fourth.”

At 10:30 p.m., a Jungles resident heard groans from an alley and ran to investigate, finding Officer Allen lying in a pool of blood. He died a few hours later without regaining consciousness. According to the coroner’s report, Office Allen “came to his death … by a blow from a heavy, blunt instrument, probably a brick, crushing his skull, the same being the result of a conspiracy…” Indeed, a bloody brick containing hairs from the victim was found near Allen.

The coroner’s report went on to name the conspirators as “Roy Kelly, James Clouse and Harry Hill and others unknown to us — and said act was felonious.”

But justice proved more elusive than the coroner’s conclusions. Four days later, the local newspaper reported that Kelly, a notorious bootlegger, Clouse, a “frequent offender,” and Hill, “a well-known figure in the Jungles,” had been arrested for Allen’s murder. But the district attorney failed to secure sufficient evidence and the three men were freed.

The case grew colder for almost a century. Then in 2005, author McLellan, at that time a captain of the patrol division, made contact with the bearer of an old letter. Penned by a woman who was a young girl in Fort Collins in 1907, the letter recounted a story told by the girl’s father: “They had an exclusive club … where a few picked men could play … So they had whiskey and a policeman found out … and came nosing.” The letter goes on to pin the murder on “a banker.”

Could the letter refer to a different crime? McLellan says no. “Throughout the history of the FCPD, two officers gave their lives in the line of duty, and the only case that remains unsolved is the murder of Joseph Allen.”

Author McLellan welcomes inquiries and any new information regarding this case. Email him at tomactam@msn.com.