The lawyer for former Deputy Chief of Staff Bridget Kelly sent a letter Monday to the committee's lawyer saying she would not comply because the information demanded "directly overlaps with a parallel federal grand jury investigation."
Christie fired Kelly last month after it was made public that she told a Port Authority of New York and New Jersey official in an email that it was "time for some traffic problems in Fort Lee."
Former Christie campaign manager Bill Stepien made a similar argument last week and is also not complying with a subpoena.
Kelly is now the third key figure in the scandal to decline to cooperate with the committee by invoking the 5th Amendment.
She joins former Port Authority official David Wildstein, who took the 5th during a hearing last month, and Stepien, who notified the Legislature Friday he would be taking the 5th.