The program indicated that they were "probably" killed due to de-pressurization, but it sounded to me like that was just wishful thinking.
The pictures of McAuliffe's parents just after the explosion really get to me. Her mother looks like mine and it tears me up every time I see that video and see the confusion and anguish in their faces.
de-pressurization would not have killed the crew.
The crew possibly, but not certainly, lost consciousness in the seconds following orbiter breakup and they would not have been regained it before water impact. Yet, it was undetermined if the cabin actually lost pressure, so it was possible they were conscious for the near three minute descent. Slaming into the ocean is what killed them.
"When Challenger broke up, it was traveling at 1.9 times the speed of sound at an altitude of 48,000 feet. The crew module continued flying upward for some 25 seconds to an altitude of about 65,000 feet before beginning the long fall to the ocean. From breakup to impact took two minutes and 45 seconds. Impact velocity was 207 mph, subjecting the module to a braking force of approximately 200 times the force of gravity. Any astronauts still alive at that moment were killed instantly."
http://www.space-shuttle.com/challenger1.htm