The killer is Richard. Forensic expert, Lincoln Rhyme wrote an article years ago stating Marcus (Richard) planted evidence at a crime scene, for which he received six years in jail. He committed the murders and planted clues because he wanted to test how good a dectective Lincoln actually is. At the end, Richard kills Thelma and then goes after Lincoln. When Richard is about to stab Lincoln, who is a quadraplegic, Officer Amelia Donaghy arrives and shoots Richard.