Three days previously, in the first launch attempt, the launch was stopped at T−7 seconds, just prior to the SSMEs' ignition sequence, due to a senior console operator manually triggering a cutoff in the countdown. It was later determined that the console operator, monitoring the hydrogen gas concentration in the Space Shuttle's aft compartment, where the three SSMEs are located, was fooled by the gas detector's purge cycle, which generated a dangerously high, but spurious, reading in the last seconds of the countdown.