Movie suggests a murder (Spoiler)
Michael was murdered through the use of *contaminated* crack/cocaine. First we get the view of a third party observing Michael buying crack from a guy. Toward the end we see the same guy passing the drug to Michael. But when Michael walks away he watches him with some concern. Also I read on (shudder)wikipedia that there were sudden deaths of several Dallas attorney agents investigating the GPO case. Madness!