Newark, N.J. • What began as a mysterious series of traffic jams at the George Washington Bridge exactly two years ago has mushroomed into a high-level corporate scandal that can't be good for Republican Gov. Chris Christie's struggling presidential campaign.

United Airlines CEO Jeffrey Smisek and two other top executives abruptly resigned Tuesday amid a federal investigation into the possible trading of favors between the airline and David Samson, the Christie-appointed former head of the Port Authority of New York and New Jersey, the powerful agency that runs area tunnels, bridges and airports.

When Samson was in charge at the Port Authority, United resumed direct flights to the South Carolina airport near his vacation home. Around the same time, United was pressing for concessions from the agency, including a new hangar at the Newark airport, rent reductions and a commuter rail-line extension that would connect the airport directly to lower Manhattan.

No one has been charged in the case. A spokeswoman for Samson on Wednesday said only that Smisek's resignation "is a United Airlines matter." A Port Authority representative had no comment.

The investigation was an offshoot of the so-called Bridgegate case, the scandal that has cast a long shadow over Christie's White House hopes.

Three Christie allies — his former deputy chief of staff and two former top executives at the Port Authority — were charged last spring with closing lanes and engineering all-out gridlock at the foot of the nation's busiest bridge in September 2013 to exact revenge on a Democratic mayor who declined to endorse Christie's re-election bid. Christie has denied any knowledge of the plot.

Federal and state authorities expanded the bridge investigation to examine possible wrongdoing in the handling of billions of dollars in public-works projects undertaken by the Port Authority.

Samson headed Christie's gubernatorial transition team and has long been a key adviser. He resigned in 2014 after the Port Authority was implicated in the bridge scandal.

Asked about the scandal on CNN on Wednesday, Christie said that he has focused in his public career on "making sure people hold to certain legal and ethical standards in their conduct in office."

"I have stood by that standard my entire career, and I hold everyone who works for me to that standard, and if they don't hold to that standard, then they're fired," he said. "And that's the way it works. And so we'll see what happens with this situation."

Apart from putting the bridge scandal back in the headlines, it is unclear what effect the United part of the case will have on Christie's presidential campaign.

The governor has largely failed to distinguish himself from the crowded GOP field, and despite frequent visits to New Hampshire and dozens of town hall meetings, he has yet to crack low single digits in the state in most polls.

A number of political operatives in New Hampshire said Wednesday that unless something comes to light to directly link Christie to wrongdoing at the Port Authority, the latest turn is unlikely to play a role in the campaign.

Fergus Cullen, a former chairman of the New Hampshire GOP, said 'Bridgegate' is already "baked into the cake" among the voters.

Federal prosecutors in New Jersey have subpoenaed, among other things, records pertaining to the little- used flight to Columbia, S.C., which was discontinued by United days after Samson resigned.

While Samson was chairman, the Port Authority approved contributing $10 million toward construction of the hangar, and voted to go ahead with plans to extend the PATH commuter train line.