It looked like an explosion but actually wasn’t. The center tank disentegrated. What looks like an explosion is the fuel escaping at that high velocity and altitude. The strain completely pulled the shuttle apart and thw crew cabin was blown clean away. The astronauts were likely alive all the way down.
Kind of a matter of definition. Later finite-element analysis indicated that the shuttle heat shield experienced (briefly) a force of five million pounds from the expanding gas cloud. That was way more than the shuttle had been designed to withstand; its weight was something like 110 thousand pounds (not counting payload bay contents), so 5 million pounds of thrust would have imparted an acceleration of like 45Gs.
It wasn't a brissant explosion, as you would have with nitroglycerine, but LH2 and LOX burn very fast when combined.
They were alive. I knew a couple of the Navy salvage divers who found the crew cabin and the investigation showed ample evidence the crew was alive until impact. Everything was buried and sealed in an unused silo and the records of the findings by the divers and investigators was tossed in a locked drawer.
The original design had provisions to use pyrotechnics to separate the crew capsule from the bulk of the shuttle, that option was removed as it was thought the the risk of having pyrotechnics was greater than the benefit of having that provision for the small fraction of the flight where it would be needed.
And the parachutes that were part of the system?
They were removed as well, they were quite heavy and at $10,000/kg to orbit, and given that there was simply no way they'd be needed without being able to separate the crew compartment...
And yes, the crew survived the disintegration of the shuttle, several emergency oxygen packs were manually activated on the way down and one first aid kit was broken into.
*sigh*
The investigation showed they were alive.