The congressional inquiry into the government handling of pre-9/11 intelligence opens today behind closed soundproof doors on the Capitol’s top floor. The inquiry will begin with briefings by a staff of 30 aides who have read more than 100,000 pages of documents and interviewed nearly 200 witnesses.

Former CIA general counsel Jeffrey Smith told The Washington Post the hearings are at least as significant as the Iran-Contra hearings and the Church Commission. Idaho Senator Frank Church’s select committee in 1975 uncovered details of CIA involvement in plots to assassinate Fidel Castro and aid a coup against Chilean President Salvador Allende. It revealed FBI, Army and National Security Agency spying on dissident groups and such prominent people as Martin Luther King, Adlai Stevenson and Supreme Court Justice William O. Douglas. The inquiry at that time prompted restrictions on the intelligence agencies’ surveillance practices and created the congressional oversight committees that will lead today’s inquiry. But the committees meeting today are now poised to undo nearly three decades of those restraints.

Meanwhile, the revelations over what the intelligence agencies knew before September 11 just keep coming. The CIA told the FBI 18 months ago that one of the 9/11 hijackers was attending a meeting of suspected terrorists in Kuala Lumpur, Malaysia, and that he had a type of visa which should have drawn suspicion, this according to a senior U.S. intelligence official based on CIA email records. Monday’s disclosure contradicts repeated assertions by senior FBI officials that bureau headquarters had no information about Khalid al-Mihdhar until three weeks before the September attack. As recently as yesterday afternoon, FBI officials said the CIA’s failure to share information had possibly resulted in a missed opportunity to unravel the September 11 plot. The FBI has even assembled a chart showing how agents could have uncovered the plot with more information from the CIA about al-Mihdhar and another hijacker.

Government officials revealed on Monday the CIA was alerted to alleged 20th hijacker Zacarias Moussaoui in spring 2001 by an informant who knew Moussaoui only by an alias. The agency didn’t link the two names until well after September 11. As a result, the CIA’s original background check on Moussaoui came up empty after he was arrested at a Minnesota flight school a month before the suicide attacks.

Egyptian President Hosni Mubarak says Egyptian intelligence warned U.S. officials about a week before September 11 that al-Qaeda was in the advanced stages of executing a significant operation against a U.S. target.