Salou, Spain ? A few months before 9-11, several al-Qaida operatives slipped into this Spanish beachside resort, blending in with the tourist crowds to hold what investigators increasingly suspect was a pivotal rendezvous in planning the terrorist plot.

A senior Spanish official with extensive knowledge of the investigation said authorities now believe the mid-July gathering was the last face-to-face encounter between hijack ringleader Mohammed Atta and his al-Qaida liaisons in Europe.

The main details of the conspiracy to crash commercial jets into the World Trade Center and the Pentagon were largely in place by the time the meetings were over, the official said, speaking on condition of anonymity.

Around 700 pages of detailed evidence have been compiled in two separate investigations by the national police and Spain’s Civil Guard and handed over to the FBI, according to the Spanish official and others in Washington.

A senior U.S. law enforcement official said the FBI has bolstered its presence in Spain in recent days and is engaged in several ongoing operations.

American investigators believe al-Qaida conducted more than a dozen mid-to-high-level meetings in Spain in the five years before the attack and the U.S. official said investigators are looking for evidence of financial transactions the group might have made here.

“It seems as though Spain, among other places, has been considered an operational safe-zone for al-Qaida a place where operatives can meet and plan … attacks and secure funds,” the U.S. official said.

Spanish and FBI detectives have been in Salou and other resorts on Spain’s Gold Coast, scouring hotels, travel agencies, car hire firms, regional airfields and an amusement park for traces of the men.

They have interviewed about 20 witnesses who say they may have seen the al-Qaida plotters during their rendezvous, according to the Spanish official.

Suspected participants in the meetings, possibly at a safe house, are believed to have included Atta, Ramzi Binalshibh, hijackers Ziad Jarrah and Marwan al-Shehhi and two other Arab men.

Based on nearly a yearlong investigation, authorities have pieced together much of Atta’s travels but little on what he and other suspects discussed in connection with Sept. 11.