What is Known: Mohamed Atta, who was a hijacker aboard Flight 11 that crashed into the North Tower of the World Trade Center, Marwan al-Shehhi, who was aboard Flight 175 that crashed into the South Tower of the World trade Center and Ziad al-Jarrah, who was aboard Flight 93 that crashed in Shankesville, traced back to a common venue: the Al-Quds mosque in Hamburg, Germany. Atta, 33, had an Egyptian passport, al-Shehhi, 23, had an United Arab Emirate passport and Jarrah, 26, had a Lebanese passport. Their actions began to take a common course by late 1999 when the three men all reported their passports missing and obtained new passports to replace them. By doing so, the trio erased any visas or other tell-tale signs of previous travels.

With their "clean" passports, the trio applied in mid May 2000 to the US consulate in Berlin for tourist visas to the United States. When they received their visas, they bought air tickets to the United States.

Al-Shehhi was the first to arrive in America. He landed at Newark airport on May 29, 2000 and, moving to south Florida, rented quarters for himself and Atta. He then opened up a joint bank account at the Florida Sun Trust Bank.

Atta meanwhile proceeded by bus from Berlin to the Czech Republic. After being turned away at the border for lack of a proper visa, he returned to Berlin, got a transit visa, and returned to Prague. The next day he flew from Prague to the Newark and joined al-Shehhi in Florida.

Jarrah was the last of the trio to arrive. He went from Berlin to Munich, then caught a plane to Atlanta, Georgia on June 28. He then joined Atta and al-Shehhi in South Florida.

The trio took flying lessons on small single-engine planes between July and December 2000 Atta and al.-Shehhi at Huffman Aviation in Venice, Florida, and Jarrah at Florida Flight Training Center in Venice, Fla.

Meanwhile $109,000 flowed in to the Atta and al.-Shehhi bank accounts rom an United Arab Emirates bank account in the name of "Mustafa Ahmed."

Although the Hamburg trio did not fly jet planes or airliners, in December 2000, Atta and al.-Shehhi had three hours apiece of training on a jet aircraft simulator in Opa-Locka, Florida and purchased flight deck videos for Boeing jetliners. (Jeddah had no known training on jet simulators or aircraft).

In January 2001, they made short trips abroad lasting less than 2 weeks. Jarrah flew to Dubai Airport in the United Arab Emirates, Atta flew to Madrid.

In April, Atta made a 2 day journey to Prague. In Prague, he was observed meeting with Ahmed Khalil Ibrahim Samir al.-Ani, the second Secretary of the Iraqi Embassy in Prague, and an Iraqi intelligence case officer. That summer, Atta made a trip to Spain, and two trips to Las Vegas.

In late August, Jarrah bought Global positioning equipment and cockpit schematics for Being 757s. Atta bought a first class ticket on Flight 11; al.-Shehhi bought a first class ticket on flight 175 and Jarrah bought a first class ticket on Flight 93.

Atta and Shehhi forwarded most of the remaining money in their bank account to the account in the United Arab Emirates and proceeded to Boston. On September 10th, Atta then rented a car and drove to Portland with one of the Jeddah conspirator with a false passport. The next morning, Atta and his companion, took a connecting flight to American Airlines 11, which three of the other Jeddah conspirators boarded from Boston.

Al.-Shehhi, meanwhile, in Boston, joined four Jeddah conspirators on Flight 175.

Jarrah went to Newark, where he joined three Jeddah conspirators, on Flight 93.

Here is what is not known:

1. Why Atta made his side trip to Prague before coming to the USA in June 2000. Presumably, he needed to meet someone in person just before going to America. But it is not known who he saw in Prague, or why the side trip took on such urgency that he had to return to Berlin by bus to get a visa.

2. Who sanctioned and provided the $109,000 that was funneled through the United Arab Emirates account to the Hamburg Conspirators. The identity of "Mustafa Ahmed" has not been determined.

3. The purpose of Atta, al.-Shehhi and Jarrah trips after completing their flight courses in December 2000. Did they change their mission?

4. The purpose of Atta liaison with Al.-Ani in Prague in April 2001. Did Al-Ani provide Atta with instructions, documents of any assistance? Did Atta have any further contact with Al.-Ani after he was expelled from the Czech Republic later that April?

5. How did Atta, Al-Shehhi and Jarrah join with the 13 Jeddah conspirators who arrived in America between May and July? Someone, somewhere, must have arranged their meeting. The relationship between the Jeddah and Hamburg conspirators is unknown. What was their order of battle and who was in command?

6. The purpose of Atta subsequent 12 day trip to Spain in July 2001.

7. The purpose of Atta's two brief two-night trips to Las Vegas. There was no observed meeting between Atta and any other hijacker in Las Vegas. If he met anyone on these trips, who was it?

8. The purpose for Atta trip from Boston to Portland on September 10th. Atta and his companion chose to pass through the security checkpoint in Portland instead of Boston. Were weapons or tools smuggled through for him in Portland by another party?

9. The job of the Hamburg trio in carrying out the attack. None of them had any known training on an actual jet (Atta and Al-Shehhi had a maximum of 3 hours in a simulator). There is therefore no reason to assume that they, as opposed to the Jeddah crew, piloted the airliners on flights 11, 175 and 93. Had they been involved in a prior plot? Did they perform preliminary intelligence work on the Boeing 767s and 757s, which would account for their cross-country first class flights tro Las Vegas and elsewhere? Did they organize the hotels, driving licenses, false identities and other logistics for the Jeddah conspirators? Were they on the planes or did they provide their boarding passes to others?