New Jersey Governor Chris Christie, a top Republican contender for the 2016 US presidential nomination, says he has fired a top aide who allegedly deliberately created traffic jams last year to punish a political rival.
Mr Christie apologised and said he was "embarrassed and humiliated" by her apparent actions .
The scandal is being called the biggest test of his political career.
The massive traffic snarl-up at one of the world's busiest bridges, which links New Jersey and New York City, caused hours-long back-ups for commuters and others as children started the school year.
The US attorney in New Jersey announced today he is investigating the shutdown "to determine whether a federal law was implicated".
Mr Christie, a blunt, outspoken governor who has worked to create a pragmatic, bipartisan image and contrast it with a bitterly divided Congress, early on dismissed questions about the lane closures with jokes and denied that either he or his staff had been involved.
After emails and texts appearing to link the aide to the closures were reported yesterday, he cancelled public appearances and several hours later issued a statement saying he was "outraged and deeply saddened" by the revelations. He said he was misled by a key aide and he denied involvement.
Mr Christie told reporters today that he has fired that aide, saying she lied to him.
"We fell short of expectations," Mr Christie said, adding that he was "stunned by the abject stupidity" shown in the incident. He said he is responsible for what happened and said he would to go to the affected town to apologise.
"I had no knowledge or involvement in this issue, in its planning or execution," he added.
The revelations raise new questions about his leadership on the eve of his second term designed to jumpstart his road to the White House. In less than two weeks, his second inauguration is planned in the shadow of the Statue of Liberty. He also faces a national travel schedule as chairman of the Republican Governors Association.
Democrats at the national level swiftly circulated the news of the scandal, calling it more evidence that the potential Republican candidate for president is a bully. Some conservative Republicans who have been stung by Mr Christie's comments in the past joined in.
Even if Mr Christie navigates this challenge quickly, it will almost surely come back to haunt him in a presidential run, said Republican operative Hogan Gidley.
"I don't necessarily think it's Christie's policy that's going to ultimately catapult or sink his campaign; I think it's his personality," Mr Gidley said.
The email and text messages were obtained by The Associated Press and other news organisations amid a statehouse investigation into whether the huge traffic backup 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," Christie deputy chief of staff Bridget Anne Kelly wrote in August in a message to David Wildstein, a top Christie appointee on the Port Authority of New York and New Jersey. The authority is in charge of the heavily used George Washington Bridge.
"Got it," Mr Wildstein replied. A few weeks later, Mr Wildstein closed two of three lanes connecting Fort Lee to the bridge.
The messages do not directly implicate Mr Christie in the shutdown.
"What are these people doing?" asked a baffled former New Jersey Republican Governor Tom Kean, whom Mr Christie has often described as a mentor. "The closer to the governor this is, the more harm that it's going to do."
Fort Lee Mayor Mark Sokolich, a Democrat, called it "appalling" that the traffic jams appear to have been engineered.
Mr Sokolich said the gridlock put people in danger by holding up emergency vehicles, and he added that those responsible should resign.