Fox News video. Someone on ground happened to have a video camera and happened to be pointed towards crash site (a portion of country side with nothing of other interest- they were not taking pictures of Mickey Mouse and accidently captured the crash) prior to impact and continued filming after impact
My interpretation is that the initial sighting was of debris with the naked eye and that due to the quality of the phone camera used did not show this debris very clearly. Only the subsequent explosion from the impact of the main section is more obviously seen.