Not exactly flying chase at the time.
Not a test flight at the moment of the collision but the flight did start as a test flight with the pilot in the F-104N (NASA 813) flying chase during that test flight. After completion of the scheduled test points, the two aircraft rendezvoused with the other aircraft for the GE photo shoot.
I wonder how long it was before someone fessed up to the Pentagon brass that this was in fact not a test flight.
The investigation (which was wrapped up quickly in August 1966 due to substantial political pressure) revealed that approval for the photo shoot was arranged at local level with some in the ranks having the authority to reject the photo shoot but did not do so, and not following proper procedures. Proper procedures for approving such a mission should have gone from local through higher headquarters.
The investigative board ultimately concluded that the position of the pilot in the F-104 relative to the XB-70 left him with no good visual reference points for judging his distance. Therefore, a gradual movement in any direction would not have been noticeable to him. An inadvertent movement of the F-104 placed it in a position such that contact was inevitable. The length of the precision formation may have been a factor. Cloudy weather had extended the flight time and forced the formation to move to a different area than had originally been planned. Other air traffic in the area created distractions (a B-58 in a supersonic corridor at higher altitude caused other members in the formation to report traffic in sight by seeing its contrail). Accidents are usually the result of several factors, so no one knows for sure what happened that day.