Faint Black Trail extending out in front of high altitude aircraft

Observed two high altitude aircraft with the usual contrails.  One aircraft was behind and to the left of the other aircraft; however both were flying in the same westerly direction (290 degree heading, I estimate). Since they were both in the same general area, they caught our eye on this bright clear day. They were not directly overhead, but maybe at 70 degrees above horizon, if you consider 90 degrees is directly over head. We were looking at them in a southwesterly direction. Both I, my wife, and 12 year old daughter observed a faint black trail extending a great distance from the front of the lead aircraft. It was in perfect trajectory with the heading of this aircraft. Observed this strange trail for only about five seconds and then the trail was gone.
I cannot tell you if they were military type of aircraft or not, since they were so high up.