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When Mayor Bill de Blasio acknowledged this month that his administration had kept secret that a former chief of staff was forced to resign after two women accused him of sexual harassment, the mayor said he was shocked and sickened by the aide’s conduct.

But it turns out that barely a month before he was hired by the de Blasio administration, the aide, Kevin O’Brien, had been fired from his previous job as a senior adviser at the Democratic Governors Association in Washington for similar reasons, the association confirmed on Monday.

Mr. O’Brien had been a senior staff member for Gov. Steve Bullock of Montana, a centrist Democrat who has been mentioned as a possible 2020 presidential candidate. Mr. Bullock served a one-year term as chairman of the governors association in 2015 and he sent Mr. O’Brien to Washington to be his representative at the organization.

When Mr. O’Brien left the governors association in December 2015, Mr. Bullock was among those who knew the reason: A woman employed at the association had accused Mr. O’Brien of sexually harassing her, and an investigation had backed her up.