blow-by of the o-rings had been seen on early shuttle flights, and a pattern was noticed by the engineers who built the boosters: more blow-by happened in cold temperatures. the company that built the boosters, thiokol, had been working on a fix but had not told nasa even though the o-rings were "criticality 1" parts, meaning that if they should fail to do their job, the rocket would be destroyed. criticality 1 parts were also not allowed to rely on a backup part for safety - this meant that the backup o-ring in the booster was not good enough to keep the astronauts safe.