American and European investigators are trying to piece together the conspiracy that led to the September 11 terrorist attacks on the United States. The focus of that probe is on Germany, where three of the suspected hijackers lived and studied during much of the 1990s, and where investigators face challenges as they continue the investigation.

The prime focus of U.S. investigators in Europe is to shed light on the preparations for the attacks on New York and Washington, which they say were planned in the German port of Hamburg. German and U.S. authorities agree that three alleged hijackers - Mohammed Atta, Marwan al-Shehhi and Ziad Samir Jarrah - who were based in Hamburg before leaving for the United States early this year did the crucial spadework for the attacks in the German city. But, they say, other Hamburg residents were also involved in the plot.

The investigation is now centered on three men of Arab origin who were known to have spent time with the alleged hijackers during their time in Hamburg. One is Said Bahaji, a 26-year-old German citizen of Moroccan descent, who was once Mr. Atta's roommate. U.S. officials say Mr. Bahaji was the cell's logistics expert. The others are Ramzi Binalshibh, a Yemeni, and Zakariya Essabar, a Moroccan. Their roles in the plot are less clear. U.S. officials believe all three fled Germany about a week before the attacks in the United States. Mr. Bahaji is thought to have fled to Afghanistan - proof, says one U.S. investigator, of his ties to Osama bin Laden's al-Qaida network, which is headquartered there. The whereabouts of the other two men are unknown.

Although Mohammed Atta is portrayed as the key organizer of the September 11 attacks, counter-terrorism expert Jonathan Stevenson, of London's International Institute for Strategic Studies, says he is convinced that the Egyptian-born suspected hijacker got a lot of help from outside his immediate circle.

"There is little doubt in my mind, from what I know of the way al-Qaida operates, that the attacks in the United States on September 11 had to have been coordinated and financed by somebody in addition to Mohammed Atta," said Mr. Stevenson. "On the other hand, the way the cell system works in al-Qaida, it's possible that a group of which Atta was the head could have come up with the plan and some of its operational details, subject to approval and support from the al-Qaida war council in Afghanistan."

Some German police officials insist there is no conclusive proof that Mr. Atta's group in Hamburg had ties to the al-Qaida network. Steven Simon, a former counter-terrorism official with the U.S. National Security Council during the Clinton administration, says the dimension of those links is still unclear, and will emerge as the investigation progresses. But he says he has no doubt about the connection between Mr. Atta and al-Qaida.

"My impression is that the Muhammad Atta group ... went off to conduct this operation, probably under the command of some person who has not yet been, at least securely, identified by investigative authorities, someone who sort of had command and control, so to speak, over the operation, and who probably checked things out just before it took place, and then got out of the country, got out of the United States," said Mr. Simon.

With the hijackers' three accomplices safely out of Germany, the investigation there is now focusing on other, less central figures, who had close contacts with the Hamburg group. And there has been friction between German and American investigators.

One German police official says FBI agents dispatched to Germany contacted and interrogated potential witnesses before the German detectives charged with making inquiries. He says that kind of FBI action hurts chances of getting convictions, because it gives witnesses an idea of what investigators suspect. For his part, a U.S. official complains that German police procedures are too bureaucratic and slow.

Matthew Dunn, a security expert with Control Risk Group in London, says the problem between the Germans and the Americans stems from a lack of familiarity with each other's methods.

"The German police have complained that the FBI sidestepped procedures in Germany and [had] gone to approach and interview suspects before informing the German police. I'm sure that's not so much jealousy as a desire to achieve results as quick as possible," suggested Mr. Dunn. "I think, in general, cooperation between different agencies will be better, as they are aware that these groups do operate across national borders, and the only way to stop them is by better cooperation between governments and between agencies."

The Germans have also been angry at U.S. suggestions that Hamburg was the main center for planning the September 11 attacks. They admit most of the preparations took place in their country, but insist that the final planning was done in the United States itself and that authorities in both countries failed to see the danger signals.