N.J. Gov. Chris Christie let go of a key adviser on Thursday as a federal investigation loomed over his administration’s ordering of lane closures that crippled traffic on the George Washington Bridge last September, allegedly as an act of political retribution.

U.S. Attorney Paul Fishman announced the probe after email messages made public Wednesday showed the shutdown was ordered by Christie’s deputy chief of staff, Bridget Anne Kelly, and orchestrated by David Wildstein, an executive at the Port Authority of New York and New Jersey appointed by Christie. Wildstein resigned his post last month.