HAMBURG, Germany, Nov. 15 - Shortly after Sept. 11, when the United States sent out urgent inquiries for leads on the suspected hijackers, German officials quickly traced three of the ringleaders to an apartment in this affluent northern port.
It was an important break. But the names and address "Marienstrasse 54" suddenly seemed familiar, German investigators say, and then they realized why: in 1998 and 1999, they had those same men and that same apartment under surveillance because of suspected links to an operative for Osama bin Laden.
When nothing came of the surveillance, it was abandoned. Yet during that period, the investigators now ruefully admit, Mohamed Atta and his colleagues used the apartment as a base to plan the Sept. 11 attack.
"We have learned a lot in the meantime," confessed a senior German government official, referring to the group, now known as the Hamburg cell.
Among the lessons, investigators say, is just how easily terrorist plotters can blend in with innocent foreign students in large Western cities like Hamburg, making their detection so challenging even when the police have them directly in their sight. The Hamburg cell members were indistinguishable from hardworking Arab and Muslim students seeking only to gain skills and education. For the tenants of Marienstrasse 54, the aim was to turn the West's techniques into the means of its own destruction.
The tenants under surveillance included not only two of the actual hijackers but three other plotters who slipped into Pakistan and probably Afghanistan in the days before the attack. Today, American and German investigators say, those three are among the most wanted men on earth. They know as much as anyone alive about the plot, and they are dangerous, likely to attack again.
In recent days, German and American officials have been meeting in an effort to pin down everything they know about the three fugitives. One, Ramzi Muhammad Abdullah bin al- Shibh, a Yemeni, whose real name may be Ramzi Omar, may well have been the missing 20th hijacker who failed to participate in the Sept. 11 attacks because he was denied a visa, the F.B.I. recently said.
Another was perhaps even more important, the logistical brains behind the cell. He is Said Bahaji, a student of electrical engineering who fooled German intelligence as well as his father-in-law.
Osman Kul, whose daughter, Nese, married Mr. Bahaji at a local mosque in October 1999, said he had little grounds for displeasure when he learned of the couple's plans. Over coffee in his living room, Mr. Kul said he did not know until much later that some of the seemingly nice young men at the wedding were radical Muslim students involved in the plotting for Sept. 11.
Mr. Kul, the son of Turkish immigrants to Germany, admits that he would have preferred a son-in-law of Turkish descent. But he was pleased when Mr. Bahaji, now 26, and Nese, who just turned 21, had a son, Omar, last spring.
Since Sept. 11, Mr. Kul, like German investigators, has had a different view. Mr. Bahaji is thought to have obtained apartments, organized financing and communications, and helped the German hijackers apply for visas to the United States. Though he was probably never intended to participate directly in the hijackings, his activities show how seamlessly the group blended into student circles in working class neighborhoods.
"I see now that he was an actor," Mr. Kul said, "a splendid actor."
Mr. Kul last saw his son-in-law in early September. At that time, Mr. Bahaji announced that he was going to Pakistan for computer courses.
At the meeting, the roles of Mr. Kul's daughter and son-in-law appeared reversed, he said. His daughter, an active young woman who loved snorkeling and horseback riding, was clad in lengthy garments and a veil in the Muslim tradition; Mr. Bahaji, apparently in preparation for his trip, had shaved his thick beard.
Investigators traced his route to Karachi, Pakistan, via Istanbul, Turkey, but there the trail was lost. Except for an e-mail message sent to his wife shortly after his arrival in Pakistan, he has not been heard from since.
The man who almost led investigators to the hijackers, Mamoun Darkazanli, a dapper 43-year-old native of Damascus, Syria, came to the attention of the German police in 1998 after the arrest near Munich of Mamdouh Mahmud Salim, who was later extradited to the United States, where he faces trial in New York for terrorism.
American officials had placed Mr. Salim on a terrorist watch list early in the summer of 1998. In August of that year, bin Laden operatives set off truck bombs outside two American embassies in Africa, killing more than 200 people. A month later, Mr. Salim went to Germany.
Mr. Salim's cellphone had Mr. Darkazanli's phone number programmed into it. Mr. Darkazanli had signing rights for Mr. Salim's account at the Deutsche Bank
Mr. Darkazanli told the police the deals were legitimate business. Reached by phone recently, he refused to answer questions, saying he was acting on the advice of his lawyer.
Mr. Bahaji, born in Germany, spent his childhood here and in Morocco. He served in a tank unit of the German Army before settling in Hamburg to study. Mr. Bahaji may have been responsible for the plotters' computer literacy.
Thorsten Albrecht, who rented the apartment to the group, described how the plotters equipped each of the three bedrooms with a table and computer hooked up to high-speed data lines. After marrying, Mr. Bahaji left the apartment on Marienstrasse and moved with his wife into a two-room, ground-floor apartment nearby that still has his name on the mailbox.
The German authorities say, however, that Mr. Shibh, 29, from Yemen, lived in the apartment after Mr. Bahaji left. Like his roommates, though not Mr. Bahaji, Mr. Shibh was supposed to go to the United States, learn to fly and take part in the September attacks. In the summer of 2000 he applied for a visa to study flying at the Florida Flight Training Center, in Venice, Fla., where another of the Sept. 11 pilots, Ziad al- Jarrah, studied.
On Aug. 15, he transferred $2,200 from his account at Citibank in Hamburg to a branch of the West Coast Bancorp
The letter terminating the Marienstrasse lease was also signed by the third missing man, Zakariya Essabar, a 24-year-old Moroccan, whose apparent ambition to perish in the September attacks was also thwarted. Abdelghani Mzoudi, another Moroccan student, who signed the letter and Mr. Atta's last will and testament, recently described Mr. Essabar as something of a headache for his fellow tenants: he never wanted to help clean the kitchen, had visitors at odd hours and was very secretive.
On the Thursday after the hijackings, the German police raided an apartment in a three-story brick building on the eastern edge of Hamburg, looking for Mr. Shibh, but they found only a shaken Algerian couple, Ali Hermouche and his wife. Mr. Hermouche, in an interview, said he met Mr. Shibh in July in a cafe, and Mr. Shibh told him he was a poor student in need of a place to stay.
Mr. Hermouche offered him the keys to his apartment while he and his wife took their August vacation. When they returned on Sept. 8, Mr. Shibh had been there, but he had left the key in an envelope in the mailbox as they had agreed. Mr. Shibh has not been seen since.