The history of large-scale terrorist bombings of American targets prior to 9-11 is instructive. The bombings of the World Trade Center (1993), the US military residences at Khobar Towers in Saudi Arabia (1996), the US embassies in Tanzania and Kenya (1998) and the USS Cole in Aden, Yemen (2000) had a common modus operandi. All four attacks divided the conspiracy between those who gathered intelligence, tested the security and set up the operation and those who placed and detonated the bomb. The former set of advance men were in each case retracted or escaped, the second latter conspirators— the bombers— either were suicidists killed in the attack or left behind.

Consider the first World Trade Center attack in 1993. The advance man, and organizer, was a Baluchi operative named Ramzi Ahmed Yousef. After he arrived in New York, he recruited a team of expendable conspirators to rent a van, load a 1,500 pound bomb in it, park it in the underground garage under the north tower of the World Trade Center and detonate it. This latter group was not provided any means of escape and were quickly caught by the FBI. The only exception was Abdul Rahman Yasin, who managed to flee to his home country of Iraq after the FBI mistakenly released him.

Ramzi Yousef was a very different story. He was on a plane to Asia soon after the bomb exploded. He easily escaped because an escape route had been prepared for him. He had his plane ticket, money and, most important, a false Kuwaiti identity "Abdul Basit Mahmood Abdul Karim" that he had not previously used in America. This legend had been intricately constructed, as Laurie Mylroie documents in her book, Study of Revenge. The real Abdul Basit Mahmood had vanished during the Iraq invasion of Kuwait in 1990. Since Kuwait's Interior Ministry files were in the custody of Iraqi's intelligence service, it was possible to insert Ramzi Yousef's fingerprints in Karim's passport application and other records. So if Yousef's false identity had been checked on his departure from the US in 1993, it would have been confirmed by Kuwait's passport records.
After undertaking another bombing assignment in Manila— the destruction of 11 US trans-Pacific airliners— which was aborted, he was arrested in Pakistan in 1995 and deported to the US.

The truck bombing of Khobar Towers in 1996 that killed 19 Americans followed a similar pattern. Security camera tapes, police reports and other evidence showed that it had been under sophisticated surveillance for nearly a year before the attack. There were also methodic probes. In one incident, a car actually tested the strength of the barrier by slowly ramming it. After the conspirators had assessed the target, a truck-bomb with 20,000 pounds of TNT was assembled with explosives imported from abroad. The team of local Saudis who were recruited to deliver and detonate the bomb were soon caught and imprisoned in Saudi Arabia. By that time, the members of advance team who, according to the Department of Justice, "supported and supervised the attack," had been retracted from Saudi Arabia. Subsequently they were identified as Iranian operatives.
In the case of the near-simultaneous bombing of the US embassies in Tanzania and Kenya, resulting in 213 deaths, the conspiracy also used a division of labor. There were teams of bombers in each country, who were expected to die in the attacks, and advance teams who left prior to the attacks. Mohamed Sadeek Odeh, who had help prepare the truck bomb in Kenya, testified after he was captured in Pakistan, that he had left Kenya the day before the attack and traveled to Karachi under an assumed name with a fake passport. So did his co-conspirator, Fahid Msalam, who purchased equipment and vehicles for the Tanzania attack (and was never apprehended). They both were on the same Pakistani International Airways flight.

The attack on the USS Cole, a US warship in Aden Harbor, which required long preparation, technical skills with C-4 shaped-charge explosives and sophisticated operational intelligence, also used a division of labor. The advance men had prepared another suicide boat attack ten months earlier on the American destroyer, USS Sullivans, which had to be aborted. They then rented a house, built a fence around it and recruited two suicidists to drive a fiberglass boat loaded with C-4 next to the USS Cole. The suicidists died in the attack, while the advance teams, who according to the Yemen investigation had been divided into two men cells, escaped. All investigators found at the house used to stage the attack was several names on fake identity cards and on documents in homes and cars, which turned out to be untraceable.

Why wouldn't the 9-11 conspiracy use a similar division of labor? Like the other attacks, the 9-11 aerial bombing required two separate missions: 1) advance agents, capable of conducting months of reconnaissance, probing and other intelligence-gathering activities for the mission; and 2) expendable assault teams capable of taking over Boeing 757s and 767s, navigating them to pre-selected targets, and crashing the planes into them.

The advance parties' had the task of blending into the American air- travel scene for a sustained period of time without drawing attention to themselves or otherwise arousing suspicions. To do this, they had to be fluent in English, have sufficient social skills and possess plausible cover stories that would allow them to ask questions about airliners and boarding procedures. The teams began arriving in America more than a year before the attack and established their residences, bank accounts and driver licenses. Like the USS Cole reconnaissance teams in Yemen, they usually operated in pairs. They enrolled in flight schools which, among other things, would provide a plausible cover for their interest in airliners. They acquired flight manuals, diagrams and videos of the targeted planes from mail order houses, and booked time on simulators. They traveled back and forth cross the country as first class passengers on Boeing 757s and 767s, according to the FBI investigation, where they could scout the access to cockpits, locate cockpit keys and observe flight crew behavior after the planes took off. As first-class passengers with flight school credentials, they presumably could also ask questions of the flight crew. In January 2001, members of these advance teams returned to the United Arab Emirates and other foreign countries where they could be de-briefed and then returned to the US.

The groups trained to take control of Boeing airliners and convert them into flying bombs required a different set of skills. They had to be able to work together as a team in rapidly gaining entry to locked cockpits. They had to have the weapons to overpower the flight crews before they could give warning of the coordinated hijackings. They had to take over the flight controls, turn off the transponder, re-set the coordinates and then maneuver the airliners to their targets. None of these skills were taught at the flight schools in Florida, Arizona, California and Minnesota.

Most of the assault team members who arrived in America in June and July of 2001 had identities that had been prepared for them, as had rooms, gym memberships, funding and other logistics. In late August, nineteen hijackers were given boarding passes on flights departing from Logan, Dulles and Newark Airports.

The 9-11 planners did not sacrifice all the conspirators aboard the planes. A number of those involved in the advance planning (for example, Ramzi bin al-Sibh, Khalid Sheikh Mohammed, Zacarias Essabar and Said Bahaji) escaped to Pakistan prior to the attack.

Some of conspirators doing the reconnaissance work in America similarly might have been retracted, even if their names were used to buy the plane tickets (for the fatal flights) on the Internet. All they would have had to have done was to give their boarding passes and fake photo ID to members of the assault team, change their identities and vanish.

Consider Mohamed Atta, a well-educated architect with an advanced degree who had traveled in the United States, Europe and the Middle East. Atta had helped prepare the attack for 15 months through reconnaissance, logistical and liaison work. He had flown on numerous first-class flights on American and United Airlines, attended flight schools in Florida, cased airports, purchased flight videos, rented rooms and cars, transferred money and, in late August, made reservations for American Airlines Flight 11 that departed from Logan Airport in Boston. He traveled to Boston on about September 8th. Then, on or about September 10, 2001, just about ten hours before the planned hijacking of Flight 11, which was taking off from Boston, Atta left Boston. Accompanied by a dark-set man traveling under a stolen identity, he drove in a rented car to Portland, Maine, and checked into a motel.
But why? If he had been the designated pilot of the suicide team that planned to hijack Flight 11 before 8 AM in Boston, he would have made the suicide mission that had been planned for months contingent on the commuter flight from Portland not being delayed by poor weather or other circumstances. It also meant that he had to go through two security checkpoints— one in Portland, and a second one in Logan— since the commuter plane from Portland would arrive at the south side of Terminal B, which was at that time separated by a parking lot from American Airlines Flight 11 on the north side. So, returning to Boston would entail for the mission an extra search and chance of detection. There is no actual evidence that he returned: Logan Airport had no security cameras where he arrived so he was no photographed there and, though messages were left for him on his voice mail, he did not retrieve them.

In Portland, Atta allowed himself to be photographed by a number of security cameras. The last such picture was at 5:58 AM on September 11th at Portland International Airport. Later tht day, his suitcase, containing his will and airplane videos, were recovered in Portland by the FBI.

He could have flown to Boston and been on the suicide flight or he could have escaped that morning under a different identity— and some other conspirator then could have used his boarding pass to get on Flight 11.

There is no doubt that the 9-11 conspiracy had access to multiple identities and false documentation. The dark-set man who accompanied Atta to Portland used the passport of Abdulaziz Alomari to gain entry to the US in June 2001. It had been stolen in 1995 in Denver from the real Abdulaziz Alomari, who was then a Saudi engineer, studying at the University of Denver and who reported the identity theft at the time. Other hijackers had the identities of Saudi who had years earlier gone to fight in Chechnya, and were presumably killed. So they could have been of any nationality and have been trained pilots.
Since the conspiracy had purchased false photo IDs for boarding the planes, the tickets could have been purchased under any convenient aliases. By electing to use names of members of the advanced party who would be sought by American authorities after the attack, it assured that they would be presumed dead. Whether they were actually died aboard the planes or used their presumptive deaths to mask their escape is not known.