I was flying a Frontier flight from CO to Dallas FW. The flight departed CO at about 4:50 PM. We had arrived at our cruising altitude about 10 minutes before seeing the object. I had taken several pictures of the clouds and was continuing to observe them when I saw an odd shaped object leaving a strange looking vapor trail. My phone was on the tray so I grabbed it and snapped a few pictures. The object was moving from East to West when I first saw it. It was also dropping in altitude. As it started to pass beyond my view (we were moving East while the object was moving West) it made a sudden turn downward and appeared to make a roll-like maneuver to assume a West to East path. At this point it was moving close to parallel with our flight. It held this path for about twenty seconds before suddenly disappearing. When looking at the attached photos note they are in sequence. They have not been altered in any way. The object appears to be small - the pictures are taken with an iPhone 5. The object is in every image, once you find it you'll be able to track it in the photos. Image 5005: Fairly centered in the image. This is the first photo. The object is moving at about a 30 degree angle towards ground on a western track. Image 5006: Right hand side of the photo. The object made a sharp turn straight downward. There are two vapor trails visible. This was taken as the object made it's downward turn and roll changing flight direction from west to east. Image 5007, 5008, and 5009: The image can be seen almost center of the photo. It tracked almost the same speed as our flight for about 20 seconds before suddenly disappearing.