I'VE been recruited to help solve one of Denver's most notorious crimes. On December 18, 1922, a Federal Reserve Bank delivery truck was being loaded with money outside the Denver Mint when three men pulled up in a black Buick and jumped out, firing sawn-off shotguns. Fifty guards returned fire but the robbers still managed to get away with $US200,000. One of them, Nicholas Trainor, was killed in the gunfight but the two others were never identified.

The case remained unsolved for 12 years until the Denver police suddenly announced they'd worked out who was responsible. Conveniently, all the gang members had either since been killed or were already in prison.

No one was ever charged in relation to the crime and the case was officially closed on December 1, 1934.

Today, we've been instructed to be at the entrance to platform two in Denver's impressive Union Station at 10am. Just as the clock ticks over the hour, an agitated-looking woman comes clattering down the underpass wearing a 1920s-style blue sequin dress and carrying a battered brown suitcase.

She explains that she is Mrs Florence Thompson, girlfriend of Nicholas Trainor, and that she never got her share of the loot. Of the $200,000 that was stolen, only $80,000 was recovered by police, so she wants us to track down local hoodlum Tommy Bell to help trace the rest of the cash.

Someone asks her what's in it for us. "I don't shoot you," she replies, quick as a flash, before handing us our first clue and disappearing back up the underpass. "And remember now," she shouts back, "no lollygagging around."

And so begins a Denver walking tour with a difference. The first clue directs us to the city's LoDo (Lower Downtown) district, a fashionable area near Union Station that has been revitalised with cafes, restaurants and bars. We've been told to find a renowned independent bookstore and then track down a copy of a book on the robbery that will contain our next clue to the mystery.

Other clues have us rummaging behind rubbish bins and water fountains, unravelling coded messages and even approaching a so-called "tourist" to take our picture (the clue is on her camera - just don't tell anyone I told you).

Along the way we get to check out many of Denver's famous sights: the mint where the robbery took place; the impressive Colorado State Capitol Building (which is exactly one mile (1.6 kilometres) above sea level, hence Denver's nickname of the Mile High City); the Denver Performing Arts Complex (the second-largest performing arts centre in the world) and the Colorado Convention Centre, which, bizarrely, has a 12-metre-tall sculpture of a blue bear outside.

What really brings this tour to life are the characters you meet along the way. Outside the mint, we come across Paul Clar, a Denver Express reporter who was a witness to the crime. He regales us with the gory details of what happened on that fateful December day and points out the bullet holes that are still in the mint's stonework. We also meet the "ghost" of Nicholas Trainor, who reveals what happened after the robbery, and an FBI agent, who tells us of the struggles police had tracking down the suspects.

The intriguing part is that we never know where we're going next or who we're going to meet. On several occasions, we find ourselves eyeing someone innocently sitting in a park or lingering on a street corner, thinking that he or she is our next clue.

Ensuring all the clues are where they need to be and that everyone is in the right place at the right time must be a logistical nightmare but the tour is executed flawlessly. It's one of the most ingenious city tours I've come across and, by the end of it, we've covered not only many of the city's main sights but we've also learnt a fair bit about its history.

Of course, the big question is what happened to Mrs Thompson? Did she track down Tommy Bell? Did she get her share of the loot?

That's not for me to say. But if you find yourself near platform two on Denver's Union Station at 10am, there might just be someone there who knows.

To read the complete article, see:Case of the Mile High mystery(www.smh.com.au/travel/case-of-the-mile-high-mystery-20100520-vhqx.html)

To read an earlier E-Sylum article, see:OLD HARVE BAILEY CAPTURED(www.coinbooks.org/esylum_v08n35a14.html)