Black "condensation" trail stayed at pace with jet for several minutes and then shot forward and all dissipated.

February 8, 2008, mid afternoon--flying from San Diego to Denver.  
While flying over Arizona, noticed a black trail, sort of a carbon trail as opposed to a condensation trail with the point staying even with the jet in which I was riding.  
I and my seat mate watched for several minutes as it seemed to keep pace exactly with our jet, I glanced toward my seat mate who immediately looked past me and brought my attention back to the carbon trail (or whatever), it had suddenly shot forth and was well ahead of our plane.  Within seconds the entire trail seemed to dissipate. 
Whatever was causing the blackish smoky trail was far enough out (south) from the plane that we could not detect an object, just the trail.
At first, we wondered whether the witnesses might have been looking at the shadow cast by the aircraft's contrail, but if the object moved away from the passenger plane, we do not see how a shadow could explain the phenomenon.  PD