ALOU, Spain, April 28 — Last July 9, an important Al Qaeda member arrived at a small airport near here on a tourist flight from Germany. Unlike millions of other foreigners who flock here to the Gold Coast in search of sun and fun, Ramzi Muhammad Abdullah bin al-Shibh slipped into a shadow world.
Sometime during the next several days — Spanish and American authorities remain uncertain precisely where and when — officials believe that Mr. Shibh crossed paths with Mohamed Atta, the Egyptian-born suspected mastermind of the Sept. 11 attacks.
Those officials believe that the two men met face to face and that their conversation marked a significant moment in the plot. Mr. Atta, who had once shared an apartment with Mr. Shibh, went to considerable trouble to reach this small coastal city in early July.
He flew from Miami to Zurich, where he purchased a knife, and then to Madrid, where he rented a car and drove halfway across the country to Salou.
Mr. Shibh's activities during his week in Spain, and whether he delivered money, instructions or plans for other operations, remain a central enigma of the Sept. 11 attacks in New York and Washington. But American and Spanish officials are persuaded that the 29-year-old Yemeni is one of the few people with direct knowledge of the conspiracy hatched in the apartment he shared with Mr. Atta in Hamburg, Germany.
Just six days before the Sept. 11 attacks, Mr. Shibh flew to Pakistan from Germany. He has not been heard from since, but he is believed to be still alive.
Mr. Shibh was one of the few people to move money for Al Qaeda and investigators believe that he had been picked to be a pilot of one of the hijacked jets, a plan that went awry when he could not get a visa to enter the United States.
Solving the mystery surrounding Mr. Shibh's role in the attacks and his disappearance a few days before Sept. 11 is described by law enforcement authorities here and in the United States as essential to unraveling the intercontinental puzzle surrounding the attacks and possibly stopping future Al Qaeda operations.
Spanish intelligence officials disclosed Mr. Shibh's July visit for the first time in an interview, saying that they were certain only that he came and left. Two senior officials say the strong presumption is that he was here to meet with Mr. Atta, who arrived in Madrid late on July 8 and drove the next morning to this region on the Mediterranean coast.
Mr. Atta, who flew the first plane to crash into the World Trade Center and is suspected of being the plot's ringleader, was an old friend of Mr. Shibh. The two men were part of a circle of radical Islamists in Hamburg who have been tied to Al Qaeda.
Mr. Shibh's disappearance in early September has frustrated investigators, who have mounted a global manhunt for him. More than seven months into the world's largest criminal investigation, huge uncertainties persist about the most basic aspects of the plot.
Among the gaping holes are where and when the attacks were planned, how the hijackers were assembled and if someone outside the core group coordinated their action.
From what is known about Mr. Shibh, it appears that he could provide many of the answers. Authorities believe he was involved in the planning in Hamburg, beginning in late 1998, and had extensive contact with Al Qaeda members across Europe. After his unsuccessful attempts to join the hijackers at flight school in Florida, he wired money to them from Germany.
Most recently, Mr. Shibh's phone number was linked to a German suspected of involvement in the bombing of a synagogue in Tunisia, and he was also tied to an Al Qaeda financier in Spain.
"We want him desperately," a senior F.B.I. official said.
Spain was a natural staging area and meeting place for Al Qaeda. As legal residents of Germany, Mr. Shibh and Mr. Atta could enter Spain or any European Union country without a visa or passport, and usually without more than a cursory glance at an identity card.
No one is certain why they came to this area. It may have been because the region is overrun with tourists and they would have blended in. But the police said it could have been to meet people living in the area. Whatever the reason, they were comfortable enough to use their own names because neither was on a watch list and careful enough to cover their tracks.
Unseen, Alone and Efficient
Spanish investigators trying to reconstruct Mr. Shibh's movements said he covered his tracks and avoided mistakes. He dressed in Western style and passed unnoticed on the edges of European society, just as the 19 hijackers did in the United States. He traveled alone and avoided known radicals, who were presumed to be under surveillance by the local police.
The F.B.I. has been working with Spanish investigators to recreate the July visits of Mr. Shibh and Mr. Atta. As recently as mid-March, the Spanish police and a Spanish-speaking F.B.I. agent canvassed shops and hotels along the coast.
"The F.B.I. agent was trying to tie Atta and al-Shibh together," said Antoni Banyeres, owner of the Montsant Hotel in Salou, where Mr. Atta spent July 17.
Another hotel owner said authorities clearly suspected that Mr. Shibh and Mr. Atta met with several other men. He said the police showed him photographs of Mr. Shibh and Marwan al-Shehhi, who flew the second plane into the World Trade Center and whose possible presence in Spain in July was first reported by the newspaper La Vanguardia.
"They are sure three men and maybe more were here to meet with them," the hotel owner said.
Senior Spanish intelligence officials confirmed that they are investigating the possibility that other plotters were in the region, but they said they have come up empty: No one has recalled seeing Mr. Atta and Mr. Shibh together.
Registration records showed that Mr. Atta stayed at the Casablanca Playa Hotel in Salou on July 16 and at the Montsant the next night. But the police do not know where he spent the rest of his time.
His former roommate has proved virtually invisible. The police have found no trace of Mr. Shibh in Spain except records of his arrival at nearby Reus airport on July 9 and his return to Hamburg on July 16, two days before Mr. Atta headed back to the United States.
Germany has charged Mr. Shibh with helping plan the attacks. The F.B.I. director, Robert S. Mueller III, called him the 20th hijacker and the Justice Department considered charging him along with Zacarias Moussaoui, the man often described as the 20th hijacker.
A senior F.B.I. official said that Mr. Shibh was believed to have traveled to Pakistan and gone into Afghanistan after he left Spain in September. On Jan. 17, the Justice Department released a videotape found in Afghanistan in which he pledged to die for the jihad, but authorities have not said if they know when or where it was made.
Hallmarks of Discipline
More than seven months after the attacks, the mysterious life and confounding disappearance of a man at the center of the plot provide a case study in the ability of Al Qaeda to plan operations unobtrusively, communicate secretly and behave with discipline, attributes all the more dangerous after indications from the Tunisian synagogue bombing that the network may be reactivating.
A slightly built man, about 5 feet 7 inches tall, Mr. Shibh was born on May 1, 1972, in the Hadramaut region of eastern Yemen, according to a rental application he filled out in Hamburg. Little is known about his upbringing, though friends said he worked in a bank for a time, experience that may have helped later.
He was 23 when he arrived in Hamburg by ship on Sept. 27, 1995, and applied for political asylum, claiming to be a Sudanese refugee named Ramzi Mohammed Abdullah Omar. The application was rejected and his appeal was denied in December 1997. An arrest warrant was issued for Mr. Omar in May 1998.
By that time, Mr. Shibh had re-entered Germany, using his own name and a Yemen passport. It is unknown where he obtained his visa.
In Hamburg, he loaded boxes at a computer company and attended a vocational school. A fellow student told Der Spiegel magazine that Mr. Shibh "always received failing grades in math because he slept or read the Koran under the table."
An F.B.I. agent testifying in Virginia in an unrelated fraud case against a man who knew Mr. Shibh described the Yemeni as a cleric who told friends in the mid-1990's that he wanted to participate in the war in Bosnia.
On Nov. 1, 1998, he moved into a small, three-bedroom apartment at 54 Marienstrasse, three blocks from Hamburg-Harburg Technical University. His roommates were Mr. Atta, an Egyptian studying at the university, and Said Bahaji, a 26-year-old German citizen of Moroccan extraction.
Thorsten Albrecht, the landlord, said the men had told him they wanted to share the place to save money, and all three presented legal residence cards.
"We ate a meal together in the office, filled out contracts and signed them," he said. "It was a three-man team, so that no one could have said that one or the other was the leader."
Among the mysteries is how the roommates met. Some authorities suspect that it was at the Al Quds Mosque in Hamburg, a gathering point for militants. Others suggested that someone assembled and directed the team.
One frequent visitor was Ziad al-Jarrah, 25, who flew the plane that crashed in rural Pennsylvania. Another was Mr. Shehhi, 23, who flew the second plane that hit the World Trade Center.
Mr. Shibh may have met two other hijackers. In early January 2000, Khalid al-Midhar and Nawaq Alhazmi attended a meeting in Kuala Lumpur, Malaysia, with a suspected Al Qaeda member and an unidentified third man, American and Malaysian officials said.
Mr. Midhar and Mr. Alhazmi, both Saudis, were in the cockpit of the flight that crashed into the Pentagon. Malaysian authorities said the unidentified person at the meeting was from Yemen and some reports have identified him as Mr. Shibh, though some investigators doubt it was he.
On May 17, 2000, two weeks before Mr. Atta and Mr. Jarrah left for the United States, Mr. Shibh applied for a visa at the American Embassy in Berlin. The application was denied, as was a second application the next month.
Signs of Desperation
Despite the setbacks, Mr. Jarrah tried to enroll Mr. Shibh in flight school in Florida that August, and Mr. Shibh arranged to wire money to the school, according to the court documents.
Apparently desperate, Mr. Shibh flew to Yemen in September 2000, American investigators said, and applied again in Sana, the capital. He was rejected. Back in Germany, his final application was denied on Oct. 25, 2000.
American intelligence officials said that Mr. Shibh had been denied a visa on routine grounds, not because of any suspicions that he was a terrorist. They said the bar is high for Yemenis because many overstay their visas, so while Mr. Atta and others could get student visas, Mr. Shibh was refused.
At this point, Mr. Shibh may have given up on joining his colleagues. On Dec. 2, 2000, he flew from Hamburg to London. Once more he seems to have become a ghost, leaving no record of where he went or with whom he met.
But American investigators suspect that he delivered instructions to Mr. Moussaoui, a French-Moroccan who had attended an Al Qaeda training camp in Afghanistan in 1998. Unconfirmed reports placed Mr. Atta at the camp at the same time.
On Dec. 9, as Mr. Shibh returned to Hamburg, Mr. Moussaoui flew to Pakistan where investigators said he received $35,000 in cash for flight school in the United States. After a trip back to London, Mr. Moussaoui was taking flight lessons in Norman, Okla., by the end of February.
Records in the Moussaoui case show that in July 2000 Mr. Shibh had already wired money to Mr. Shehhi, who was in Florida and shared a bank account with Mr. Atta. A second wire transfer followed on Sept. 25.
Investigators believe that the money came from Al Qaeda through the United Arab Emirates. In late July 2001, Mr. Shibh received $15,000 in two wire transfers from a man in the U.A.E. identified in the court records as Hashim Abulrahman.
On Aug. 1, investigators said, Mr. Shibh took a train to Düsseldorf and sent a money order to Mr. Moussaoui in Oklahoma from a bank at the train station. Court papers show that he used the name Ahad Sabet. On Aug. 3, he sent another money order to Mr. Moussaoui from the train station in Hamburg, using the same false name. The amounts totaled almost $15,000.
The use of a false identity and money orders increased the secrecy of the transactions. American and Spanish investigators speculate that security was heightened because Mr. Shibh had learned the date of the attacks in the United States at a meeting with Mr. Atta in Spain two weeks before.