The guy at the very beginning of the movie who worked on Lincoln Rhyme's (Denzel Washington) heart monitor by the name of Richard is the killer.

His real name is Marcus, and he used to be a forensic investigator.

You don't really see him again until the very end when just before he tries to kill Denzel, he explains to him why.

It turns out that years earlier, Denzel Washington wrote an article accusing him of planting evidence, framing, and sending six innocent people to prison, which consequently sent Marcus to jail.

Years later, he gets out of jail and begins a series of "copy-cat" murders from the "Bone Collector" book about actual murders. These are an attempt to lead Denzel to him.

After killing the chief of police and Queen Latifah, he attempts to kill the paralyzed Washington.

He drags Denzel out of the bed and right before stabbing him, Anglina Jolie comes to the rescue, shooting the killer just before he can bring the knife down into Denzel's chest.