The 19 hijackers who took over four commercial airliners on September 11 had carefully studied gaps in security, especially weaknesses in pre-boarding screening at airports.

In the months before the attacks, the hijackers staged rehearsals, boarding U.S. domestic flights to ensure that their plan would work. Commission staff member Bill Johnstone recalls the result. "All 19 hijackers were able to pass successfully through checkpoint screening to board their flights," he said. "They were 19 for 19, 100 percent. They counted on beating a weak system."

Before September 11, regulations were in force that prevented knives more than four inches in length from being carried in aircraft.

However, smaller knives were permitted. In perhaps the most chilling part of Tuesday's hearing, a sharp silver metal folding knife called a "Leatherman" was passed down the line of commissioners, described here by commission member Richard Ben Veniste.

"We've seen this morning, this 'Leatherman' tool which contains blades of four inches, and which has the ability to lock into place," he said.

Commission investigators say that before September 11, two hijackers purchased such knives which were not later found in belongings left behind.

Former FAA administrator Jane Garvey, said aviation security had focused on threats to international flights. The hijackers, she added, took advantage of previously-held assumptions:

"These [assumptions] included the fact that politically-motivated hijackers would release passengers after landing at a safe haven, and that together with such hijackings, explosives presented the biggest threat to the system," she said. "The events of September 11 certainly challenged those assumptions. A system that had proven effective for the preceding 10 years could no longer be relied upon."

While there were growing concerns about threats to domestic aircraft, Jane Garvey said the FAA did not have credible or specific information indicating the type of attack on September 11 was being planned or even possible.

A preliminary report released by the commission this week contains information that appears to contradict accepted views of how the hijackers entered the United States.

Passports used by as many as eight of the men, according to the commission, "showed evidence of fraudulent manipulation." Investigators identified at least one suspected al-Qaida member who may have intended to join the September 11 attacks but was turned away by an alert immigration inspector.

On Tuesday, Claudio Manno, who was in charge of intelligence in the FAA at the time of the September 11 attacks, pointed to one weakness in the system at the time. "Prior to September 11, 2001 FAA did not receive a daily flow of raw reports and finished intelligence from the FBI," he said.

In the most riveting moment of this seventh hearing of the commission, recordings were played for the first time of telephone conversations between Betty Ong, a flight attendant aboard Flight 11, one of the hijacked aircraft, and the ground: "[I'm] number three in the back, the cockpit is not answering, somebody is stabbed in business class, and I think there is mace, that we can't breathe. I don't know, I think we're getting hijacked," she said.

One of the airline representatives on the ground speaking with Betty Ong was Nydia Gonzalez. "Betty, we are here to commemorate you," she said. "Your acts of courage on September 11th will never be forgotten."

Members of the commission and the public were visibly shaken after the recordings were played in the hearing room.

This week's public hearing was the seventh of the National Commission on Terrorist Attacks Upon the United States, referred to as the 9-11 Commission.

"The airlines were responsible for the safety of their passengers, and for implementing key aspects of the civil aviation security system," said Commission chairman, Thomas Keane. "On September 11, that system failed and we are charged by statute to find out why."

However, the job of the commission created by Congress in 2002, has not been easy. The Bush administration initially opposed it, and investigators have had trouble obtaining some of the classified information they have sought.