Author Vince Flynn wrote 15 novels about a spy named Mitch Rapp before he died in 2013, and they were all bestsellers despite being panned by book critics.
But for years, his fans wondered why none of his pulpy counterterrorism tales had been turned into films.
Some suspected it was because Flynn's politics made Hollywood uncomfortable.
His patriotic stories were adored by conservatives in the United States, who loved their violent, ripped-from-the-headlines plots, which often saw Rapp torture and kill Muslim terrorists.
Fans include former US president George W. Bush and right-wing commentator Glenn Beck, the latter once describing Flynn's work as "conservative porn".
This week, 18 years after the first Mitch Rapp book hit the shelves and nine years after the series movie rights were sold, there is finally a film: American Assassin, starring Dylan O'Brien as Rapp and Oscar winner Michael Keaton as his mentor.
Its director Michael Cuesta emphasises that while many view the books as politically conservative, the film, which opens in Singapore today, is not.
Speaking to The Straits Times at a recent press day in Los Angeles, he says: "I think there are people who perceive (the books) that way.
"But I feel the film is apolitical. Everyone involved with it is not trying to make a red state (a Republican-voting state) or a (President Donald) Trump movie."
It is not uncommon for the authors of such thrillers to be conservative, he adds. The late Tom Clancy, who wrote the books that became films, such as The Hunt For Red October (1990) and Patriot Games (1992), was too.
"These guys are hardcore Republicans."
But the goal should be to "tell a good story and also show both sides and something that's complex and that you can talk about", says Cuesta, 54, who has directed television series such as Homeland (2011 to present) and Dexter (2006 to 2013).
American Assassin, based on the 2010 book of the same name, is Rapp's origin story: After his girlfriend dies in a terror attack, the 20something Rapp is recruited by the Central Intelligence Agency to carry out anti-terrorism black ops.
The hot-headed wannabe agent must then be reined in by his mentor as they investigate a series of attacks, which takes them on an action-packed chase across Europe.
Many critics in the US slated the film when it was released there last month, several calling out its thin plot and dubious politics.
But Cuesta's intention was for it to be more substantive than the average counter-terrorism nail-biter. "I didn't want to make one of these rah-rah movies about patriotism and how America are the good guys and other guys are the bad guys."
Instead, he wanted to "give it more greys and complexity - some good people do bad things,and bad people do good things".
He believes Hollywood's representation of terrorists needs to change and says his film does this by not casting its Middle Eastern characters as the sole villains and not making all those characters bad.
"We have an American terrorist and, among the Iranians, there's a side that is fighting the other side. It was important for me to show both sides. It would be really irresponsible to paint it as 'We're righteous, you're wrong.' Our world is pretty complex right now," the director says.
O'Brien, the 26-year-old star of the Maze Runner science-fiction movies (2014 and 2015), echoes this in a separate interview.
Noting that he did not want to do "action for the sake of action", he says the movie looks at the motivations behind terrorism as well as the responses to it.
"Part of my interest in this project was it deals with something delicate and, unfortunately, relevant now. It's a very topical subject that you have to be very responsible with.
"I wasn't interested in doing an action movie that dealt with that with absolutely no substance," says the actor, who is dating Under The Dome (2013 to 2015) actress Britt Robertson, 27.
Keaton, 66, feels the same way. The actor - who was married to the late actress Caroline McWilliams and has a 34-year-old son - was "determined not to take a simplistic approach" to terrorism in the film, which he hopes is a "thinking person's action movie".
"I wasn't interested in saying, 'These are evil, dark-skinned people who do these horrible things, and we're all the good guys.'"
•American Assassin opens in Singapore today.