Defendants being sued for negligence in connection with the Sept. 11, 2001, terrorist hijackings will not be able to depose six FBI agents on the government’s investigation into the attacks, Southern District of New York Judge Alvin K. Hellerstein ruled Thursday.

Airlines, airline security companies and the Port Authority of New York and New Jersey asked Hellerstein to allow the depositions in the hope they might show that the government’s failure to stop the terrorists was so egregious that the attacks would have happened regardless of any negligence on their part.