In the case of the later Columbia shuttle disaster, chunks of ice fractured ...
it was foam.
Columbia was compromised by impact with a chunk (or chunks) of failed external tank insulating foam. The foam contained absorbed water that increased the mass of the foam fragment(s). I remember analyzing the high-frame rate video that was released afterwards and concluding the foam pieces were accelerated to roughly 500 MPH before striking the wing. The failure can be pinned on a new foam recipe replacing the original to meet a "green" objective. The new foam absorbed more water and had inferior adhesion to the tank. So we can blame the disaster on "green practices"...
The proximate cause for Challenger disaster was the resiliency of an inter-segment gasket on the booster. The gasket was too cold at ignition and did not conform to the seal between the segments, resulting in a rapidly eroding collimated blow-by from the solid combustion between segments. It was pointed at the base of the external tank and eventually burned through it "like a blowtorch". NASA had acquired evidence of this potential fault in previous missions but with a very self-serving and faulty risk analysis the engineering management dismissed it from concern. The concern was raised again immediately prior to the launch and dismissed with a "this one simple management trick" commonly used to keep projects on schedule despite progress-blocking issues, namely treating the concerns of the qualified engineers as anecdotally inferior to the risk of a short-term schedule lapse affecting customer relations...
I really liked Feynman's report on his conduct of the investigation, which was a significant work prior to the inspired theatrical performance he put on at the public hearing to illustrate his findings. He wrote about it in his autobiography. It is likely General Kutyna knew of or intuited the cause (he had a significant amount of experience with ICBMs and managing technical risk), but he cunningly utilized Feynman to provide an objective assessment. The Rogers Commission report was otherwise destined to be a whitewash of Nasa and industry management but for the inspired deviltry of Kutyna and Feynman....