The independent federal commission investigating the Sept. 11 attacks did not pursue a tough enough line of questioning with former Mayor Rudolph W. Giuliani during a hearing two years ago because its members feared public anger if they challenged him, according to a new book written by the panel’s leaders.

“It proved difficult, if not impossible, to raise hard questions about 9/11 in New York without it being perceived as criticism of the individual police and firefighters or of Mayor Giuliani,” wrote the chairman and vice chairman, Thomas H. Kean, a Republican, and Lee H. Hamilton, a Democrat, in their book, “Without Precedent: The Inside Story of the 9/11 Commission.”

“We did not ask tough questions, nor did we get all of the information we needed to put on the public record,” they wrote.