Wisniewski on Sunday described the letter's wording as "curiously crafted."
"The question I had: If he has stuff, it should have come to the committee," Wisniewski said.
Also Sunday, the Associated Press reported that a member of Christie's staff had resigned. Christina Genovese Renna worked under Bridget Anne Kelly, the former deputy chief of staff Christie fired last month after e-mails showed her involvement in the lane closures.
Renna was one of 20 individuals and organizations subpoenaed last month by the legislative panel investigating the traffic jams.
A statement from Renna's lawyer said she had been considering leaving the Governor's Office since November as part of a normal transition, the AP reported.
In response to a subpoena last month, Wildstein provided the Legislature with 900 pages of documents related to the lane closures, which were carried out as an apparent act of political revenge against the Democratic mayor of Fort Lee, who did not support Christie's reelection. None of those documents, Wisniewski said, directly tied Christie to the scheme.
"I'd like to see the material Mr. Wildstein's attorney talked about," Wisniewski said.
Christie's office denied the accusation and attacked Wildstein's credibility, saying Saturday in an e-mail to supporters: "Bottom line - David Wildstein will do and say anything to save David Wildstein."
The governor's supporters also went to bat for him Sunday.
Former New York City Mayor Rudolph Giuliani questioned Wildstein's motives, noting that his lawyer had asked the Port Authority to cover his legal bills. The lawyer, Alan Zegas, has suggested Wildstein would come forth with more information about the lane closures if he were granted immunity from prosecution.
The U.S. Attorney's Office is also investigating the traffic jams.
Giuliani also said on Face the Nation that Christie should remain chairman of the Republican Governors Association.