The National Transportation Safety Board determines the probable cause(s) of this accident to be:
The flight crew’s decision, due to confirmation bias, to continue the descent below the decision altitude when the runway had not been positively identified. Contributing to the accident were (1) the first officer’s fatigue, which exacerbated his confirmation bias, and (2) the failure of CommutAir pilots who had observed the localizer misalignment to report it to the company and air traffic before the accident.
CommutAir (United Express) 4933 Final Report