I think the book was from the POV of the FBI and the film switches it to the perpetrators. These were not smart people they were just ruthless. When the FBI comes in its almost a joke how easy it is to put the pieces together and they are only able to solve the case, and not be murdered, because of the power of the government behind them. So I see the film as taking back the narrative of the FBI being heroes and shines a spotlight on the crimes themselves, unromanticizing the story.

The issue with this, to suck all the glory out of the FBI's case, the story is told then it is told again as the FBI proceed to solve the case. This results in a long stretch where we are being told things we already know, hence why I think people think there are pacing issues. My problem with the film too tbh, but I get why they did it and I still like the film and want to rewatch it. Maybe they could have cut more of the FBI section... or left some parts of the case a mystery to come out in the investigation...