Two years before the Sept. 11, 2001, attacks on the United States, the ringleaders of the plot had a different destination in mind: Chechnya. But a chance encounter with a stranger on a train in Germany led the conspirators in a new direction, eventually putting them in touch for the first time with Osama bin Laden and the leadership of al Qaeda.

The 1999 episode on the German train, disclosed in the final report of the U.S. commission that investigated the attacks, is based on interrogation reports that until recently were kept secret. According to the account, a mysterious passenger -- identified as Khalid Masri, a name that has not previously surfaced in public records of the investigation -- urged the Islamic radicals from Hamburg to put off their mission to Chechnya until they could speak with a Mauritanian businessman, who in turn arranged a personal introduction to bin Laden.

The circuitous path that led the group to Afghanistan is one key piece of evidence cited by the commission in concluding that members of the Hamburg cell had no intention of attacking the United States until they were recruited almost by happenstance by the al Qaeda leadership to become the field marshals of the Sept. 11 operation.

That finding contradicts a long-held theory advanced by German prosecutors, who have argued that the Hamburg radicals wholly conceived of the plot themselves in Germany and only later traveled to Afghanistan to seek the support and sponsorship of al Qaeda.

According to U.S. and German investigators, the Hamburg cell included 11 core members and supporters who played a role in preparing for the attacks. Of those, three died during the hijackings, including plot leader Mohamed Atta; two are facing trial in Germany; two are in U.S. military custody and one is imprisoned in Syria. The other three remain at large.

The competing version of how the conspiracy began is threatening to derail Germany's attempts to prosecute two men accused of belonging to the Hamburg cell and providing assistance for the attacks. One of the suspects, Mounir Motassadeq, a Moroccan, was scheduled to go on trial Tuesday in Hamburg on 3,066 counts of serving as an accessory to murder.

Motassadeq was found guilty 18 months ago and is still the only person convicted in connection with the attacks. But the verdict was overturned on appeal, in part because of new information about the conspiracy that emerged from U.S. investigations, and a new trial was ordered.

The Sept. 11 commission's findings are based on classified reports of interrogations of Ramzi Binalshibh, a core member of the Hamburg cell who was supposed to serve as a lead hijacker but could not get a visa to enter the United States. He was captured in Pakistan in 2002 and is in U.S. custody in a secret location.

During interrogation, Binalshibh described the 1999 encounter on the German train that ultimately led the Hamburg group to meet bin Laden. His statements also led the Sept. 11 commission to reach a conclusion different from that of the German prosecutors, who alleged the plot originated in their country.

In 1999, "when the four core members of the Hamburg cell left Germany to journey to Afghanistan, it seems unlikely that they already knew about the planes operation," the commission wrote. "No evidence connects them to al Qaeda before that time."

For years, German authorities have wondered how the hijackings could have been planned in their midst without triggering any alarms. Now, the U.S. investigation has answered their question by saying the plot was not hatched here.

"This was the assumption for a long time," said Manfred Murck, a German intelligence official in Hamburg whose agency is responsible for monitoring domestic extremist groups. "In a way, we feel a little bit better that not all the responsibility is on our shoulders."

But the fresh disclosures present a severe challenge to German prosecutors in the upcoming trial. The indictment against Motassadeq charges that he played a direct role in furthering the attacks. Under German law in effect at the time, he can be found guilty of helping or belonging to a terrorist organization only if the conspiracy was planned within Germany.

A panel of judges that will hear the case in Hamburg has already requested that the Sept. 11 commission's report be included as evidence. Defense attorneys are also demanding that Binalshibh and other al Qaeda suspects in U.S. custody be allowed to testify, arguing that their accounts could exonerate their client.

The U.S. government has refused in the past to allow access to Binalshibh, citing national security concerns. U.S. officials have provided summaries of his statements to German intelligence agencies, but only on condition that they not be used in the courtroom.

German legal experts said the court will be hard-pressed to accept the argument that the interrogation reports cannot be included in the trial, given that they have already been made public in great detail by the Sept. 11 commission.

"It's simple and easy," said Ulrich von Jeinsen, a German lawyer representing families of those killed in the Sept. 11 attacks. "If the U.S. doesn't cooperate and this information remains classified, then Motassadeq will go free. It is a very critical situation."

There are signs that German authorities are bracing for a defeat. Last month, officials in Hamburg filed papers seeking the deportation of Motassadeq and another Moroccan defendant scheduled for trial later this year. The officials said they acted out of concern that the men could remain in the country if acquitted.

Kay Nehm, Germany's chief federal prosecutor, visited Washington in April in an attempt to persuade Justice Department officials to provide more access to captured al Qaeda suspects for the trial. He said the response was not encouraging.

"I met with a great deal of understanding," he said in a statement last month. "But they gave me to understand that there were restrictions which went way beyond the authority of the people I was talking to." A spokeswoman for Nehm's office called the lack of information from the United States "a black hole," but said prosecutors were still hoping for a breakthrough.

If Motassadeq is found not guilty, legal experts said, the German government will likely drop its case against another alleged member of the Hamburg cell, Abdelghani Mzoudi. He was acquitted in February on similar charges, but prosecutors have appealed.

While the Sept. 11 commission presented the most detailed account yet of how the Hamburg cell formed and allied itself with al Qaeda, much remains unclear.

For instance, German and U.S. investigators had initially placed great emphasis on the role played by Mohammed Haydar Zammar, a 300-pound auto mechanic known as a leading Islamic radical in Hamburg who had fought in Afghanistan and favored violent jihad. Investigators said he inspired formation of the Hamburg cell and provided a link to the al Qaeda leadership.

But according to the Sept. 11 commission, it was another man who pushed the Hamburg crew in al Qaeda's direction: a Mauritanian businessman named Mohamedou Ould Slahi.

Citing statements given by Binalshibh, the commission reported that Masri, the mystery man on the German train, urged the Hamburg radicals to contact Slahi when they expressed interest in finding a way to join Islamic fighters who were resisting Russian forces in the breakaway region of Chechnya.

They did, and Slahi invited them to visit him in the German city of Duisburg, where he was running an import-export business.

According to the commission, Slahi advised the men that it was difficult to slip across the border into Chechnya. He encouraged them instead to go to Afghanistan. He assisted with their travel plans and arranged for them to meet operatives for al Qaeda in Pakistan, who in turn arranged a private meeting between Binalshibh and bin Laden in December 1999.

The trip to meet bin Laden prompted the Hamburg contingent to swear allegiance to al Qaeda. It also resulted in an immediate assignment: to lead and plan the Sept. 11 hijackings in the United States.

Slahi was well known to U.S. and German intelligence agencies as an al Qaeda follower, but neither government was aware that he was living in Germany, according to the commission. He was later identified by U.S. investigators as having aided in the creation of a cell in Canada that was to carry out the plot to bomb Los Angeles International Airport during the 2000 millennium celebrations, which was ultimately foiled.

Shortly after the Sept. 11 attacks, Slahi was arrested in West Africa. He disappeared after that and is now reportedly in U.S. custody at Guantanamo Bay, Cuba.

The Hamburg court has asked the Justice Department for more information about Slahi in preparation for the upcoming trial. The Sept. 11 commission's report did not cite any statements made by Slahi.

There are other suspected members of the Hamburg cell who could provide insight into the operations of the group but who remain at large.

Zakariya Essabar, an Islamic extremist from Morocco who was close to the cell leaders, left Hamburg for Pakistan on Aug. 30, 2001, so he could notify the al Qaeda leadership in Afghanistan that the U.S. hijackings were imminent, according to the commission. German authorities have a warrant for his arrest, but his whereabouts are unknown.

Said Bahaji, a German citizen of Moroccan ancestry, shared an apartment with two members of the cell in Hamburg. He also left Germany shortly before the attacks, flying to Pakistan on Sept. 3, 2001. He is also wanted as a fugitive in Germany. Letters and e-mails that Bahaji recently sent to relatives in Germany have been traced to Pakistan by German law enforcement officials.

Special correspondent Shannon Smiley contributed to this report.