Authorities on both sides of the Atlantic have described the plots as the work of a highly compartmentalized organization whose members often weren't aware of each others' activities.
Even so, a look reveals that many of those suspected of orchestrating terror against Americans had direct and frequent connections _ a meeting in Spain, a phone number found in Hamburg, a London cleric who was a religious inspiration to terrorists in different cells.
And connected to it all, Mohamed Atta _ the suicide pilot Osama bin Laden described as "in charge of the group" of Sept. 11 suicide hijackers _ seems to have been the common thread that wove through the lives of many bin Laden agents.
Atta's travels in the last months of his life brought him in contact with some of those allegedly behind plots against the U.S. embassies in Rome and Paris, as well as a cell officials say was plotting to attack the European Parliament and other targets in Strasbourg, France. He also met an Iraqi intelligence official in Prague and is suspected of links to a group of North Africans in Spain that authorities say was planning to attack U.S. interests in Europe.
While the other al-Qaida plots in Europe were thwarted, the one Atta controlled directly _ leading a team of suicide hijackers who threw their lives away in order to take thousands more _ succeeded.
U.S. authorities, who spoke on condition of anonymity, say they have placed Atta at the center of their investigation and are working backward, retracing his steps in order to reach other potential terrorists.
What they have found is that in the months leading up to the worst terrorist attack in history, al-Qaida suspects were working throughout Europe, often bumping up against authorities and each other. In some cases, a suspect under surveillance in one country was not considered a danger by authorities in another.
THE ROME PLOT:
When the U.S. Embassy in Rome went on alert and shut down for three days last January, there was a suggestion al-Qaida may have been plotting an attack.
Four months later, Italian police nabbed Essid Ben Khemais, a burly Tunisian known as "the Saber." Italian prosecutor Stefano Dambruoso has said Ben Khemais was sent from Afghanistan to supervise bin Laden's terrorist operations in Europe. He is considered a high-ranking bin Laden associate who Spanish officials say met with Atta in Spain weeks before his arrest.
Italian investigators have said they've uncovered evidence of cooperation among al-Qaida cells in Germany, France, Spain and Belgium.
Under Ben Khemais' direction, the investigators say, an Italian-German cell plotted to bring down the heavily guarded U.S. Embassy, a 19th century building on Rome's fashionable Via Veneto. Two members of the cell were arrested shortly after the Sept. 11 attacks, including a Tunisian man picked up outside a Milan mosque referred to by the State Department as al-Qaida's "main station house in Europe."
Some time between the closing of the Rome embassy and Ben Khemais' arrest in April he met with Atta in Spain, Spanish police say. What the men discussed is not known.
THE HAMBURG CELL:
The Dec. 11 U.S. indictment of Zacarias Moussaoui, a French citizen of Moroccan descent arrested in Minnesota three weeks before the Sept. 11 attacks, shed more light on the cell Atta operated in Hamburg, Germany.
U.S. authorities allege that Moussaoui, who authorities say would have been among the hijackers had he not been arrested, followed many of the same patterns as the 19 hijackers, all of whom were named as unindicted co-conspirators along with bin Laden.
The indictment also linked Moussaoui to Ramzi Binalshibh, an alleged member of the German cell who once lived with Atta in a Hamburg apartment where they are believed to have planned the Sept. 11 attack.
U.S. investigators believe Binalshibh was meant to have been the 20th hijacker and have suggested that Moussaoui may have been picked as an alternate when Binalshibh was denied entry into the United States.
Atta's ties to Binalshibh and hijacker Marwan al-Shehhi go back years. The three worked together between 1997 and 1998 at a company outside Hamburg called Hay Computing Service GmbH, company officials said.
In October 1999, Atta and Binalshibh both attended the wedding of Said Bahaji, a 26-year-old German-Moroccan who vanished shortly before Sept. 11 and is thought to have been part of the Hamburg cell.
At Bahaji's wedding, a lavish affair at a local mosque, at least three of the hijackers and their cohorts bumped shoulders with members of Hamburg's Muslim community, including a Syrian-born import-exporter named Mamoun Darkazanli.
The Bush administration has accused Darkazanli of operating a front for al-Qaida, and Spain has named him in the indictment of Sept. 11 suspect Imad Yarkas, who comes from Darkazanli's home town of Aleppo, Syria. Atta also spent time in Aleppo during the mid-1990s while researching his dissertation on trade development there.
Despite the ties, German authorities have said they don't have enough evidence to arrest Darkazanli.
In an interview with The Associated Press after Sept. 11, Darkazanli said he met Atta at Bahaji's wedding and acknowledged a failed business dealing with a bin Laden cohort but said he was innocent of any wrongdoing.
Spanish authorities would like to know why Yarkas' Madrid phone number appeared in an address book found in Atta's Hamburg apartment after the attacks.
Did Atta meet the man suspected of recruiting and fund-raising for al-Qaida during one of his Spanish trips? Investigators can't say.
What the Spanish Interior Ministry has said is that Yarkas, who used the alias Abu Dahdah, was the "representative of the organization run by Osama bin Laden in Spain," and spent years hiring holy warriors and raising an undetermined amount of cash to pay for their operations.
On Nov. 18, Yarkas and seven others were charged in a Madrid court with belonging to al-Qaida. Baltasar Garzon, Spain's top anti-terrorism judge, said they "were directly linked to the preparation and carrying out of the attacks perpetrated by suicide pilots on Sept. 11, 2001."
While Yarkas may not have known all the details about the attacks, he knew the World Trade Center and the Pentagon were targets, according to Spain's national police chief, Juan Cotino.
And like Atta, Yarkas seems to have been connected to other suspects named in the Sept. 11 investigation, including Mohamed Bensakhria, who is believed to have been bin Laden's top aide in Europe, as well as Mohamed Boualem Khnouni, the leader of the Algerian cell now in Spanish custody.
According to the indictment, Yarkas traveled 20 times to Britain and visited Turkey, Belgium, Denmark, Sweden, Indonesia, Malaysia and Jordan as part of his recruitment drive.
As early as 1997, he began sending fighters first for training in al-Qaida camps and then to the front lines in Bosnia and Afghanistan. The indictment lists a dozen fighters under his tutelage and connections to operatives around the globe _ chiefly Omar Mahmoud Othman, a London-based preacher better known as Abu Qutadah.
THE LONDON CONNECTION:
A Spanish indictment describes Abu Qutadah as "the supreme leader at the European level of the mujahedeen," or Islamic fighters.
Jordanian authorities call Abu Qutadah a senior associate of bin Laden and last year a military court there found him guilty of conspiracy to carry out terrorist attacks on U.S. and Israeli targets. He is living in London where he has refugee status.
Abu Qutadah, a Jordanian of Palestinian descent, denies all the charges and says his mosque on London's Baker Street is nothing more than a sanctuary for worshippers.
"I am just a cleric for Islam. People talk to me from all over the world," he recently told AP.
Authorities suspect otherwise. U.S. intelligence officials have been saying since October that some of the suspects were recruited at his mosque.
Zacarias Moussaoui's brother has said Moussaoui was radicalized at the Baker street mosque.
Moussaoui also worshipped at a London mosque frequented by Richard C. Reid, the man who allegedly tried to blow up a Dec. 22 trans-Atlantic flight with explosives hidden in his shoes, said Abdul Haqq Baker, the mosque's chairman.
London was also home to Lotfi Raissi, a 27-year-old Algerian pilot who lived near Heathrow airport and was arrested on a U.S. warrant there on Sept. 21.
British prosecutor Arvinda Sambir has said Raissi trained several hijackers who flew the plane that crashed into the Pentagon, including Hani Hanjour, whom Raissi allegedly flew with on June 23 from Las Vegas to Arizona.
U.S. investigators say Atta was also in Las Vegas then with Hanjour, who would later crash American Airlines Flight 77 into the Pentagon, and three other hijackers.
THE PARIS EMBASSY:
Djamel Beghal, a 35-year-old French-Algerian, sat in a Dubai prison for two months before Sept. 11.
But after the attacks, Beghal's story about a suicide plot to blow up the U.S. Embassy in Paris caught the attention of France's top anti-terrorism judge who quickly secured his extradition.
French investigators have said that during interrogations in Dubai, Beghal said he discussed the Paris embassy plot inside bin Laden's home in Afghanistan. He later gave authorities the names of several accomplices, including Nizar Trabelsi, the alleged would-be suicide bomber.
Trabelsi, a Tunisian who once played professional soccer in Germany, was arrested in Belgium on Sept. 13. Cotino, the Spanish police chief, said it was "very possible" Trabelsi and Atta met in Spain where they both were at the same time earlier this year.
Spanish authorities have also tied Trabelsi to Algerians in their custody and to a Frenchman named Jerome Courtailler who once lived with Moussaoui and is now in Dutch custody.
The Dutch prosecutor's office said recently their suspects "tried to falsify documents that could be used to commit an assault, possibly on the U.S. Embassy in Paris."
THE IRAQI AGENT:
Atta also met last spring at a Prague hotel with an Iraqi spy who was allegedly plotting an attack on a U.S. financed radio station.
Czech authorities have said they do not know what the two discussed during the meetings but the Iraqi, Ahmad Khalil Ibrahim Samir Al-Ani, was expelled in April for "activities that are incompatible with his status as a diplomat."
Following the Sept. 11 attacks, the Czech government deployed armored vehicles around the offices of Radio Free Europe/Radio Liberty's Prague headquarters.
On Oct. 10, the Czechs announced they were in discussions with Washington about moving the headquarters out of the capital altogether because of security concerns.