Leaden skies and chilly air greeted Milt Heflin 20 years ago today when he pulled into the large parking lot outside Mission Control at Johnson Space Center in Houston.

Although space shuttle Columbia was due to return to Earth after a two-week mission, the center was quiet on a Saturday morning. When Heflin, chief of the flight director office at NASA, walked into Mission Control, he found the observation room nearly empty. While the shuttle’s seven astronauts made their final preparations to enter Earth’s atmosphere, Heflin chatted amiably with the room’s only other occupant, a mission operations chief named Ron Epps.

Through large glass windows, the two looked out over Mission Control. As the shuttle’s ground track began to cross over the United States, making its approach across the southern tier of states toward Florida, Heflin began to sense that all was not well. “I got the feeling that something was not right from the movements of the flight controllers,” he said.