The book stated that one of Mack's associates, Amir Muhammad (also known as Harry Billups), was the hitman based on evidence provided by an informant, and due to his close resemblance to the facial composite.
The book stated that one of Mack's alleged associate, Amir Muhammad (although there was never evidence that Muhammad knew Mack), was the hitman. The theory was based on evidence provided by an informant (who later recanted), and an alleged but unsupported resemblance to the facial composite.