"Our practice has always been - after we do internal decision-making about who we think is the next appropriate witness - we've always run that issue by the U.S. Attorney's Office," Wisniewski said, "just to make sure we're not doing anything that would in some way impede or interfere with what they're doing."
He said the U.S. Attorney's request was not indefinite and could be lifted.
Potential witnesses for the legislative committee, according to media reports, included Christie's political strategist Michael DuHaime; the governor's former chief counsel, Charles McKenna; and Mark Sokolich, the Democratic mayor of Fort Lee, where the lanes were closed.
Among those who have already testified are Christie's chief of staff, Kevin O'Dowd, and press secretary, Michael Drewniak.
Federal prosecutors have impaneled a grand jury as part of a criminal probe into the lane closures, which tied up traffic for four days in Fort Lee.
E-mails subpoenaed by the legislative committee and made public in January show the apparent involvement of two Christie allies who allegedly wanted to exact political revenge against Fort Lee's Democratic mayor.
"Time for some traffic problems in Fort Lee," Bridget Anne Kelly, Christie's deputy chief of staff, wrote to Port Authority official David Wildstein in an Aug. 13 e-mail. "Got it," he replied. Wildstein, who resigned in December, supplied documents to the committee, but declined to testify.
Kelly and Bill Stepien, Christie's former campaign manager, have declined to cooperate with legislators, citing their Fifth Amendment right against self-incrimination. Christie fired Kelly and cut ties with Stepien in January.
The joint panel, established in January, has sought to investigate how such abuse of power could have occurred. It has tried to fill in the blanks by issuing subpoenas for documents and oral testimony from officials in Christie's administration and at the Port Authority of New York and New Jersey, which operates the bridge.
But requests made by the U.S. Attorney's Office have presented road blocks. It previously asked the committee to delay taking testimony from Pat Foye, the Port Authority's executive director, who ordered the lanes reopened and suggested whoever closed them may have broken the law.
A spokeswoman for the U.S. Attorney's Office said she couldn't comment on nonpublic communications.
Former federal prosecutors said additional sworn testimony taken outside a government proceeding could create headaches for the government.
"If the U.S. Attorney's Office is conducting its own investigation, they are undoubtedly concerned about other sworn testimony being taken which can be used at a later time to impeach the credibility of the witnesses," said George Parry, a former attorney for the U.S. Department of Justice in New York.
"Every time a witness gives a statement, their story always changes. It's human nature," said Parry, now a lawyer with Davis, Parry & Tyler in Philadelphia.
The legislative investigation "just muddies the water" for the U.S. Attorney's Office, which has substantial powers, he said.
Another former federal prosecutor said that if a witness provided two versions of sworn testimony under oath, that could create "more opportunity for defense counsel to fuzz it up."