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.

“The Port Authority Office of Inspector General has referred the matter to us, and our office is reviewing it to determine whether a federal law was implicated,” said Rebekah Carmichael, a spokeswoman for Fishman.

The lane closures were allegedly in response to the refusal of Fort Lee’s Democratic mayor, Mark Sokolich, to support Christie for reelection last year. In one email exchange, Kelly said, “Time for some traffic problems in Fort Lee,” and Wildstein replied, “Got it.”

Christie held a press conference Thursday announcing Kelly’s firing, just as Fishman was announcing the investigation. Christie, who previously had denied any involvement of his administration, was contrite, apologizing for the closures but saying Kelly’s deception had kept him in the dark.

Christie called the action “a rogue political operation” and said he was embarrassed and humiliated by the conduct of members of his administration.

The emails became public due to a subpoena issued to Wildstein by the state Assembly Transportation, Public Works and Independent Authorities Committee, which is also investigating the affair.

On Thursday, after his attempt to quash a subpoena calling him to testify at a hearing was denied by a state judge, Wildstein took the Fifth Amendment, prompting the committee to hold him in contempt.

Christie also announced on Thursday that he had asked his reelection campaign chairman, Bill Stepien, to withdraw his name from consideration for the job of state Republican Party chairman—and to give up a consultancy with the Republican Governors Association, which Christie chairs—in light of released emails showing Stepien and Wildstein discussed the closures.

The shutdown of two of the three access lanes to the bridge from Fort Lee put the town in gridlock for four days, hampering paramedics and making it difficult for children to get to school.

The U.S. Attorney’s Office is likely to explore whether the closures interfered with interstate commerce, which could give rise to a criminal charge, says Alain Leibman, a former assistant U.S. attorney now at Fox Rothschild in Princeton.

Another subject of the probe may be whether the closures constituted a conspiracy to deprive Fort Lee’s mayor and residents of their civil rights, says Darren Gelber, president of the Association of Criminal Defense Lawyers-New Jersey.

Gelber notes that in the past, the case might have been prosecuted as honest-services fraud under mail-fraud and wire-fraud statutes. But in 2010, the U.S. Supreme Court held, in Skilling v. United States, that honest-services fraud is too vague to form the basis of a prosecution unless a bribe or kickback was involved.

However, a former assistant U.S. attorney who spoke on condition of anonymity said the facts of the bridge case might still be considered honest-services fraud if Wildstein’s Port Authority appointment can be shown to have been a quid pro quo for doing the bidding of the governor’s office.

“The theory will be they placed him there for political purposes and pulled in the chit,” the former prosecutor said.

In 2008, prior to Skilling, state Sen. Wayne Bryant was convicted of honest-services fraud in connection with a sinecure job at the University of Medicine and Dentistry of New Jersey. The government alleged Bryant did minimal work but drew a salary in exchange for obtaining appropriations for the school.