TRENTON What began as a local controversy over a series of September traffic jams clogging the George Washington Bridge between Fort Lee and New York City has gained a national dimension, with a U.S. senator asking for an investigation into lane closures ordered by Gov. Christie's appointees at the Port Authority of New York and New Jersey.

Jay Rockefeller of West Virginia, the Democratic chairman of the Senate Commerce, Science and Transportation Committee, asked federal transportation officials Monday to review the unannounced lane closures, which left Fort Lee drivers in gridlock for four days.

Rockefeller also asked the Port Authority to answer questions about the closures amid speculation led by Democrats that Christie loyalists shut down the lanes as political retribution.

"I am concerned about the larger federal implications of what appears to be political appointees abusing their power to hamper interstate commerce and safety without public notice," Rockefeller wrote to Port Authority officials.

Two Christie appointees at the Port Authority have resigned after attention began to surround the lane closures on the nation's busiest bridge. The topic has spurred Statehouse hearings, subpoenas of Port Authority e-mails, and attacks by national Democrats.

Christie's top deputy at the Port Authority, Bill Baroni, a former Republican state senator, testified before a legislative committee that the closures were part of a traffic study ordered by another Christie appointee, David Wildstein. Both Wildstein, who attended the same high school as Christie, and Baroni have resigned.

The Port Authority's executive director - an appointee of New York Gov. Andrew Cuomo - has testified that he did not know of a traffic study. The Democratic mayor of Fort Lee, who was not informed of the closures, did not endorse Christie, a Republican, in his reelection bid; about 60 Democratic officials in the state did.

Christie has denied any involvement in the closures. Senate Majority Leader Loretta Weinberg (D., Bergen) said she does not think the governor ordered the lanes closed.

But politics likely came into play, Weinberg said, calling the traffic study explanation a "cover-up."

"Given those facts, and that we have never been given a rational explanation for why this was done, and that Gov. Christie's two top aides [at the Port Authority] suddenly decided to retire, you have to believe the speculation, and nothing else has come forth to explain it," Weinberg said Tuesday.

The incident, she said, appears to be "a Nixonesque Jersey trick."

With Christie predicted to run for president in 2016, the Democratic National Committee and a super PAC supporting potential Democratic rival Hillary Rodham Clinton have seized on the story, along with MSNBC's Rachel Maddow.

During an hour-long news conference Friday, Christie said he was "bothered" by the incident. "I don't like when mistakes are made," he said.

Spokesmen for Christie did not respond to a request for comment Tuesday about Rockefeller's inquiry.

A spokesman for the Port Authority, Steve Coleman, said the agency had received Rockefeller's letter and was reviewing it.

"We will be providing the chairman with a response as requested," Coleman said.

Rockefeller asked the Port Authority to respond by Jan. 15 to questions on topics including the lane closures - and which agencies officials knew of the closure plans - as well as the authority's practices and the process for coordinating decisions between New Jersey and New York state.

In urging the federal Department of Transportation to review the closures, Rockefeller said the incident "exacerbates ongoing concerns with the governance and oversight of the Port Authority."

The workings of the Port Authority are continuing to draw scrutiny from New Jersey lawmakers. On Thursday, Weinberg plans to introduce a resolution to ask Congress to evaluate the structure of the Port Authority.

Assemblyman John Wisniewski (D., Middlesex), who has been holding hearings on the closures before the Assembly Transportation Committee, has subpoenaed documents related to the closures from seven Port Authority officials - Wildstein and Baroni among them - including e-mails between the officials and Christie or members of his administration.

Wisniewski said Tuesday that Port Authority officials have indicated they are working on the documents. Responses are due Thursday.

"The big picture here is about the dysfunctional nature of the Port Authority," Wisniewski said.

He said he did not know what Christie's role was, but added, "The leader of the organization sets the tone for what is acceptable and what is unacceptable. I believe the governor has set the tone for what is acceptable by the people he appoints."

The controversy over the closures is the first to stick to Christie in his four years as governor, said Patrick Murray, a political analyst at Monmouth University.

"We don't even know whether any of this is true or not," Murray said.

He said the attention the incident has garnered is "an indication of how good the Christie camp has been at protecting the governor from any whiff of scandal."

It's also an example of the challenges Christie will face if he runs for president as a sitting governor, said Cary Covington, a political scientist at the University of Iowa.

Former governors "don't have problems cropping up on them on real time," Covington said.

As for the scrutiny surrounding the lane closures, Covington said the issue isn't on the radar in Iowa, home to the first presidential caucuses.

"It's so important to people driving between New Jersey and New York," he said. "But it hasn't reached the rest of us yet."