You will never believe this....

This is frequently the cause of confusion, but since I worked on the movie I'll offer the best explanation I have:

In Tino's screenplay, in Chris O'Donnell's performance, and throughout the initial production of the movie, the Hitman actually WAS the undercover FBI agent mentioned throughout the story. It's part of what attracted O'Donnell to the role. It was to be revealed in a filmed, but later deleted, scene where his (still living) body is found in the road by a comedian lost on his way to Vegas (played by Carlos Mencia). This is why he tape-recorded his conversation with the Chief - evidence in his investigation.

Unfortunately, the movie went through an extensive and bewildering process of re-shoots and re-cuts, and during that, the plot ended up changing so many times that many of the filmmakers themselves had trouble keeping track of what was supposed to be happening in the present cut. This lead to the necessity of a "Story Bible", written during post-production, whose goal was to provide a guideline to what would be most logically coherent, plot-wise, taking into account the original script's intricate design, the available footage, and what had been working best emotionally so far in the editing. I wrote that Story Bible, and it ended up on the DVD along with a random sampling of my notes from different cuts, which serves as a handy point-by-point comparison to how drastically a movie can go off-track.

It was one of my first experiences in Hollywood and I doubt I'll ever forget it.

Bryan's explosion of temper on that website, I have no explanation for, but hopefully this clarifies how, in a sense, both parties were correct about the existence/non-existence of an FBI agent.