On Sunday, I wrote the following article. To join the discussion, click here.

St. Paul is known for its abundance of Depression-era thugs. In the 1930s, the Barker Gang, Alvin "Creepy" Karpis, Baby Face Nelson, John Dillinger and other ne'er-do-wells found sanctuary and operated out of the gangster-tolerant city.

So when a tour business that has been exposing that historical dark underbelly for 15 years appeared to offer an online voucher for half off its popular two-hour "Saint Paul Gangster Tour," 1,000 customers snatched it up.

The only problem is that the business, Down in History Inc., housed in St. Paul's Wabasha Street caves, didn't make the offer sold through Groupon Inc. in March, according to co-owner Donna Bremer.

"People started calling us. ... They'd say, 'we just purchased your Groupon.' We said, 'we don't have a Groupon. What are you talking about?'" Bremer said.

Bremer did some online gumshoeing and discovered that a business called Twin City Trolleys had sold the vouchers.

Load the Tommy guns

The Groupon promotion had barely ended when Bremer's lawyer fired off a cease-and-desist demand to Tony Jones, manager at Twin City Trolleys, alleging trademark infringement.

Down in History first trademarked the phrase "Saint Paul Gangster Tour" in 1996 and registered it as a copyright in 1999. The trademark lapsed in 2006 and was reregistered in early April.

After a brief legal shootout in which Twin City Trolleys called into question the practical validity of Bremer's trademark, the trolley company changed its tour from "Historic Saint Paul Gangster Tours" to "Historic Twin City Gangster Tours."

"[Twin City Trolleys] did agree to make changes, but it was more to put this behind them," said Kyle T. Peterson, an intellectual property attorney at the Minneapolis law firm of Patterson Thuente Christensen Pedersen.

"The public does not associate the term Saint Paul Gangster Tours with one business," Peterson wrote in an April letter on behalf of Twin City Trolleys. "Unless that association is created, usually through extensive and exclusive sales and advertising, the public will not consider your clients' alleged mark as a source identifier."

Bremer's lawyer contends his client has created that association.

"[Down in History] has expended significant time and resources promoting its mark. ... As a result, the mark has become, through widespread and favorable acceptance and recognition, an asset of substantial value to the company," Michael M. Lafeber, a lawyer for Twin Cities-based firm Briggs and Morgan said.

The cast of characters

According to an Internet archive search, Twin City Trolleys began using the phrase "Historic Saint Paul Gangster Tours" on its website sometime between April and October 2010.

Bremer said that she has no problem with competitors offering tours, but that she took issue with the company offering one under her business' trademarked name.

Two limousine companies have entered the gangster business within the past five years, and the Landmark Center has been offering an indoor tour for 18 years. All three refer to their offerings online as "Gangster Tours."

But whereas holiday lights tours dominate the tour schedules of the two limo companies -- Renee's Royal Valet and Lee's Limousine -- gangster and cave tours are Bremer's bread and butter.

Of about 550 tours provided by Down in History in 2011, almost 200 included a gangster tour, Bremer said. The number of those tours has risen about 10 percent per year, she said.

For that reason she was willing to lighten her pocketbook by about $4,000 to fight the alleged infringement.

Bremer also took issue with the Groupon's price. Her gangster tours, guided by costumed bad guys, sell to the public for $24 per person.

Each Twin City Trolleys Groupon voucher sold for $22. The promotion listed the voucher's value at $45.

"There are very few businesses that can give you an accurate price and then reduce it by 51%. Obviously, you've got to have a profit or you'd be out of business," Bremer said.

Lisa Willis, a manager at Twin City Trolleys, said that because the company usually offers the gangster tour only to private groups, she was unable to quote a public per-guest price.

Groupon takes cover

Groupon Inc.'s role in policing trademark infringement is unclear.

Bremer contacted Groupon, but an attorney there refused to get involved. The fine print on Groupon's website places the burden of proper intellectual-property behavior squarely on the merchant's shoulders.

Meanwhile, back at the caves, Bremer, her office staff and five character actors continue to redirect confused Groupon customers to the proper boarding point for the two Twin City Trolleys tours each Sunday. "That will go on all summer," Bremer said.

Coincidentally, the boarding point is less than two blocks away, at Joseph's Grill.

Though it may pain Bremer to tell Groupon customers they didn't actually purchase her tour, in true gangster fashion she's standing her ground: Down in History has added a Sunday tour to its criminal lineup this summer.