It wasn't sabotage... Mostly, it was bad management decisions.
The Solid Rocket Boosters that were on either side of the External Tank were built in a 'stack'. Between each section, (prior to re-design), an O-ring sealed the sections together. The O-ring was temperature sensitive, and as it got cold, it became less pliable... On the morning of the launch, icicles hung from the parts of the shuttle, because it was so cold. Engineers warned that the temperature was 'out of parameters' for a launch, and recommended it be delayed, but the decision was over-ruled by management, and the countdown continued.
When the SRBs fired, the O-ring did not seal between two sections of the SRBs, and a burn-through occurred, and exhaust gases & heat began pouring onto the External Tank, finally piercing it, which caused the explosion.
It wasn't sabotage... it was worse - it was management stupidity.