Why were the tapes destroyed? Did they reveal the absence of confessions?

In their book, Kean and Hamilton wrote that they were unable to obtain “access to star witnesses in custody who were the only possible source for inside information about the 9/11 plot.”

The only information the commission was permitted to have about what was learned from interrogations of alleged plot ringleaders, such as Khalid Sheikh Mohammed, came from “thirdhand” sources. The commission was not permitted to question the alleged plotters in custody or even to meet with those who interrogated the alleged plotters. Consequently, write Kean and Hamilton, “We had no way of evaluating the credibility of detainee information” that was fed to them by third party hands. “How could we tell if someone such as Khalid Sheikh Mohammed was telling us the truth?”

The fact that video tapes of the interrogations existed was kept secret from the 9/11 Commission.