A senior aide to New Jersey Governor Chris Christie deliberately created traffic jams to punish a mayor, emails and text messages released yesterday suggested.
The controversy surrounding the rising star in the Republican party and top contender for the 2016 presidential nomination intensified.
The scandal could be especially damaging to Mr Christie who has sought to cast himself as a pragmatic leader willing to work with his political opponents.
It also tarnishes his reputation as a straight talker since he earlier denied that either he or his staff had been involved.
An "outraged and deeply saddened" Mr Christie said he was misled by his aide, and he denied any involvement in the apparent act of political payback.
The messages were obtained by the Associated Press and other news organisations yesterday.
Their release came amid an investigation into whether the huge traffic backup last year was retribution against the mayor of Fort Lee for not endorsing Mr Christie for re-election last autumn.
"Time for some traffic problems in Fort Lee," Mr Christie's deputy chief of staff Bridget Anne Kelly wrote in August in a message to David Wildstein, a Christie appointee on the Port Authority of New York and New Jersey.
"Got it," Mr Wildstein replied. A few weeks later, he closed two of three lanes connecting Fort Lee to the busy George Washington Bridge, which runs between New Jersey and New York City.
The messages do not directly implicate Mr Christie in the shutdown.
Democrats seized on the material as more evidence that the potential Republican candidate for president in 2016 is a bully.
In a statement issued late yesterday, Mr Christie said: "I am outraged and deeply saddened to learn that not only was I misled by a member of my staff, but this completely inappropriate and unsanctioned conduct was made without my knowledge."
He added: "People will be held responsible for their actions," but gave no details.
Fort Lee Mayor Mark Sokolich called it "appalling" that the traffic jams appear to have been engineered.
"When it's man-made and when it was done with venom and when it was done intentionally, it is, in my mind, the prime example of political pettiness," he said.
He said the gridlock put people in danger by holding up emergency vehicles, and added that those responsible should resign.