For more than a decade, the New Jersey attorney general’s office conducted a hard-fought legal battle to hold Exxon Mobil Corporation responsible for decades of environmental contamination in northern New Jersey.

But when the news came that the state had reached a deal to settle its $8.9 billion claim for about $250 million, the driving force behind the settlement was not the attorney general’s office — it was Gov. Chris Christie’s chief counsel, Christopher S. Porrino, two people familiar with the negotiations said.

One of those people, Bradley M. Campbell, was the commissioner of New Jersey’s Department of Environmental Protection in 2004 when the lawsuits against Exxon were filed. Mr. Campbell, in an Op-Ed article appearing in The New York Times on Thursday, wrote that “even more troubling” than the decision to settle the lawsuit were “the circumstances surrounding the decision.”

He goes on to say that former colleagues of his in the state government told him that Mr. Porrino “inserted himself into the case, elbowed aside the attorney general and career employees who had developed and prosecuted the litigation, and cut the deal favorable to Exxon.”