Only other thing I can think of, is that they ran under the bottom of a cell that Approach wasn't painting for some reason. Could have been a microburst?.... something similar to Delta at DFW in 1985. They can form fast. I was headed from DAL to MRY in a Falcon Jet that afternoon and was vectored right over the top of DFW westbound at 4,000. It was a nice sunny VFR afternoon with scattered puffy white clouds just above me and it was an unusual sight to be directly overhead DFW. When I got to MRY the line guys were all talking about Delta crashing in a thunderstorm at DFW. I couldn't believe that they had the right airport, since I'd passed overhead about 45 minutes prior to the crash. Of course, that was in August, and Delta was on the ILS.... a lot lower than Atlas.