A century ago, the Ku Klux Klan ruled Denver. Hooded hordes paraded through downtown, crosses burned on South Table Mountain, and Klan-affiliated officials controlled the mayor’s office and police department, the governor’s office, and even a U.S. Senate seat.

For all the men the Klan added to its roster, however, it couldn’t recruit Denver’s then district attorney, Philip Sidney Van Cise. When carrots didn’t work, the group turned to threats, but even in the face of a burning cross in his front yard, a car chase with Klansmen on his tail, and rumors the group planned to castrate him, Van Cise wouldn’t let the KKK keep power. During his final week in office in early 1925, he initiated a kidnapping case against Colorado’s Grand Dragon. The indictment, built on the same surveillance and undercover techniques he’d used to bust a ring of con men in Denver years earlier, was eventually dismissed, but the scandal it created helped break the Klan’s standing in the state.

Photo courtesy of the Van Cise family

With that kind of resumé, you might think that Van Cise had been celebrated as Colorado’s Eliot Ness. However, he’d been largely forgotten until his name landed on Denver’s new jail in 2010. (Van Cise shares that honor with former Denver corrections director John Simonet.) The only other place you’d likely come across anything related to Van Cise is in old news articles in the Denver Public Library’s Western History Collection—which is exactly where Alan Prendergast, a reporter and Colorado College journalism professor, found out more about the former DA. The more articles Prendergast read, the more he asked himself: Why had this man been all but erased by history?

It took a lot more digging—including convincing Van Cise’s descendants to share the prosecutor’s diary, letters, and speeches—before Prendergast felt like he finally had an answer. “Van Cise’s very presence was a reproach to those who had gone along to get along,” he writes in Gangbuster, which was published on March 28. For decades after the Klan’s reign, no one wanted to remember the cruelty they’d let flourish, even if that meant forgetting the district attorney who helped put it to an end.

Photo courtesy of Alan Prendergast

Van Cise died in 1969, more than five decades before History Colorado digitized the Klan’s ledgers and mapped its members’ home addresses, which showed the KKK’s reach extended not only to every Denver neighborhood, but also to almost every block. Prendergast believes the group’s prevalence is still something we haven’t fully confronted. “I think one of the fundamental challenges that anyone writing about the Klan in Colorado in the ’20s has is: How did it get so big, so fast?” he says. Prendergast suggests some theories in his book, including how the Klan was particularly effective in getting politicians to bow to its will. Fortunately, Van Cise proved the exception to the rule.