In fact, there had been evidence of leakage, what the engineers called "blowby, " on an earlier shuttle flight.

NPR: Challenger: Reporting a Disaster's Cold, Hard Facts

The engineers had resisted the launch, had recommended against it, citing the "blowby" in an earlier low-temperature launch and studies of the elasticity of the o-rings.

NPR: Challenger: Reporting a Disaster's Cold, Hard Facts