HAMBURG, Germany -- They arrived in Germany as students, and departed as terrorists.

The transformation of eight Arab men from devout Muslims into Islamic terrorists underlines a pattern that still troubles law enforcement authorities around the world as they confront the continuing threat of terror attacks.

The trial of one of the men, which opens Tuesday in this port city, is expected to provide a rare glimpse into the lives of some of those who have been linked to the Sept. 11 attacks on the United States.

The Hamburg cell was particularly stealthy, binding eight men, mostly in their 20s and hailing from both secular and religious Arab cultures, in a secretive mission without ever betraying their murderous purpose.

None had a history of Muslim militancy, yet eventually they found common ground in what Germany's chief federal prosecutor, Kay Nehm, calls the "hatred of world Jewry and the United States of America."

Holy war on the West

Investigators believe they came to Germany simply to study, enrolling between 1992 and 1997 at two Hamburg universities. All are thought to have been recruited after their arrival, mostly at Hamburg's al-Quds mosque.

By the summer of 1999, German prosecutors say, the men had evolved into a "closed conspiratorial group" with the goal of extending jihad, or holy war, to the West. By that October, they had settled on the World Trade Center as a target. The first group went off to Afghan training camps a month later.

In the end, three of the eight died in the Sept. 11 attacks, using their pilot training to transform ordinary airliners into weapons of mass destruction: Mohammed Atta, Marwan al-Shehhi, Ziad Jarrah.

Three more are in custody, accused of organizing logistics for the cell. Mounir el Motassadeq and Abdelgheni Mzoudi of Morocco face trials in Germany. Ramzi Binalshibh, a Yemeni believed to be the cell's chief contact with Osama bin Laden's al-Qaida network, is in U.S. custody following his arrest in Pakistan.

Two more Moroccans, Said Bahaji and Zakariya Eassabar, are on the run.

Deny Haryanto, a 32-year-old mechanical engineering student from Indonesia, helped Atta, the leader of the Hamburg cell, form an Islamic prayer group at the city's Technical University. He describes Atta as always serious and devoutly religious -- just the kind of person that experts say is sought out by al-Qaida.

"He is the most religious person I have met," Haryanto said. "His fear and awe of God made him cry while he prayed. When I came to the mosque he was always there. And when I left, he stayed."

Unknown recruiter

Magnus Banstorp, a terror expert at the University of St. Andrews in Scotland, said available information suggests an al-Qaida recruiter was at work identifying promising Muslim students in Hamburg.

"Those spotted to have potential are also subjected to psychological tests to see how cool they are under fire. The aim is to see whether they are psychologically fit to stay the course," Banstorp said.

Germany's investigation got its first break with the arrest last November of el Motassadeq, a Moroccan who in 1996-98 had security clearance at the Hamburg airport to clean aircraft. He was questioned by police immediately after the Hamburg connection to the Sept. 11 attacks was revealed, largely because of his association with Atta, who lived around the corner.

The 28-year-old el Motassadeq's trial, which begins Tuesday, is expected to provide an unprecedented view into the cell's organization as he takes the stand in his own defense.

A key element in the Hamburg cell's formation still has not been established: the identity of al-Qaida's Hamburg recruiter.

Suspicions center on a Syrian-born German citizen, Mohammed Haydar Zammar, 41, who is in Syrian custody and who reportedly told his interrogators he drafted Atta.

An acquaintance, who himself has come under suspicion, calls the story unlikely.

"He's a simple man with limited intellect, but he has a huge body and a big mouth," Abdel-Mateen Tatari, who helped bring Zammar to Germany when he was 11, said in an interview in his office north of Hamburg. "He is not an extremist, but is certainly a devout Muslim. For people to say that Zammar recruited Atta is like saying a first grader recruited a professor."

Zammar, a car mechanic, left Germany in October 2001 after being released by authorities for lack of evidence. He was later arrested in Morocco and sent to Syria.

Zammar is widely reported to have fought with Muslim guerrillas against Afghanistan's Soviet-supported communist government in the early 1990s, and later on the Muslim side in Bosnia. Tatari also disputes that.

"He weighs 150 kilograms (330 pounds) and he can hardly climb a flight of stairs, let alone fight in Afghanistan and Bosnia," Tatari said.

Tatari's properties were raided Sept. 10 on suspicions that his family has ties to an international network "of potentially violent Islamic fundamentalists" and that his men's wear companies smuggled militants into Germany. Tatari denies the allegations.

Mohammed Salah, an Egyptian who reports on radical Muslim groups for al-Hayat, an Arabic-language newspaper in London, said that the world may never know how Atta and his comrades were recruited, but that available evidence reflects the relative ease of recruiting among Europe's Muslims.

But Dia'a Rashwan, an expert on radical Islam, said he remains puzzled by the key role given Atta, who arrived in Germany at age 24 and quickly won the admiration of German teachers as a serious student.

"How could someone of bin Laden's acumen place the world's biggest terrorist operation in the hands of someone who is a complete unknown in the circles of Muslim activists anywhere?" Rashwan said.

Yet, other experts say, recruiting men without a police record worked well in the case of the Hamburg cell, giving its members freedom to prepare for the attacks in Germany, travel to Afghanistan and obtain U.S. visas without attracting notice.

Banstorp, the terror expert from St. Andrews, said if there was anything unusual about Atta, it's that he never wavered in the almost two years he contemplated slamming a jetliner into the World Trade Center.

"That impresses me the most," Banstorp said.