Describing Usama bin Laden's terrorist network as "a tree with many roots," Spain's police chief said Monday that two key suspects in Spanish custody met in Madrid shortly before the Sept. 11 attacks and knew that the World Trade Center and the Pentagon were targets.

Spanish investigators are trying to establish concrete links between 14 suspected Islamic extremists in custody here and at least one of the suicide hijackers involved in the terrorist attacks on the United States, national police chief Juan Cotino said.

"The leaders of these organizations may not have known the details, but they did know the targets that were to be attacked," Cotino said during a meeting Monday with a small group of journalists.

The Spanish probe is focusing on Imad Eddin Barakat Yarkas, the accused leader of eight suspects indicted in Spain last week on suspicion of helping to prepare the attacks, Cotino said.

According to an indictment, Yarkas' Madrid phone number appeared in an address book found in an apartment in Hamburg, Germany where hijacker Mohamed Atta lived. Police say Atta visited Spain twice this year, in January and July, but they have revealed little about what he did then or who he met.

Six Algerian suspects were arrested in Spain on Sept. 26 and suspected of membership in the bin Laden-financed Salafist Group for Call and Combat.

Cotino said police had considerable circumstantial evidence of the groups' links to the Sept. 11 attacks, echoing what Garzon said in his indictment.

Cotino said Yarkas had met several times in Madrid with Mohamed Boualem Khnouni, alias Abdallah, the alleged leader of the Algerian group arrested in September.

"The last time they met was some 15-20 days before Sept. 11," Cotino said. "They met in the street, shook hands and walked together. We have no idea what they talked about."

Police believe Yarkas is the most important Al Qaeda figure detained so far in Spain. But despite a wave of arrests, Cotino cautioned that his country was "not the nerve center" for Al Qaeda operations.

"Terrorism groups of this sort don't just center on a single country, but try to spread out," Cotino said. "There have been arrests here but there have been arrests elsewhere too."

He compared bin Laden to "a tree with many roots," that extended well into Spain.

"Undoubtedly, what we have come to call the 'dormant cells' in Spain were not in fact so dormant. They were not just here for no reason. They were here, and traveled to other places," Cotino said, referring to suspects' contacts with bin Laden representatives around Europe and trips to training camps in Afghanistan, Bosnia and Indonesia.

Cotino said police had been following Yarkas, who also used the name Abu Dahdah, since 1996 and that he is directly linked with bin Laden chiefs stationed in Europe and elsewhere.

Through intercepted phone calls, police say Yarkas refer to the Sept. 11 attacks during cryptic conversations with people who lived in the same Hamburg apartment where Atta had lived.