Trails links the US attacks to at least two European countries
Three of the suspected hijackers are known to have lived in Germany, and Spanish authorities are now investigating a theory that one of these men visited Spain in July, arriving from North Africa.
Meanwhile Europol said on Saturday that a trail led from the US to Italy, and Swiss police said they were checking whether Switzerland had been used as a transit country, acting on leads from Germany, France, Italy and Austria.
And French intelligence sources were quoted as saying that one of six people arrested on Friday in a joint operation by Dutch and Belgian police had been planning attacks on US targets in France.
After raids on eight addresses in the northern German city of Hamburg on Wednesday night, two of the suspected hijackers were named as Mohammed Atta and Marwan Al-Shehhi, both registered in Germany as from the United Arab Emirates.
It is Mohammed Atta who is suspected of visiting the resort of Salou in eastern Spain.
Spanish police are trying to establish who he may have met there.
German authorities have revealed that a third suspected hijacker with links to Hamburg is a 27-year-old Lebanese, Ziad Samir Al-Jarrah.
He is believed to have been one of 20 members of an Islamic association at Hamburg Technical University, which also included Atta and Al-Shehhi, but had also lived for a while at Bochum, in the west of the country.
Weapons seized
Police searched two apartments in Bochum on Friday, and are analysing evidence for links to other people possibly involved in the attacks.
Germany's Federal Prosecutor said on Saturday that no link had so far been found between the Hamburg group and the US's "prime suspect", Osama Bin Laden.
According to Europol Hamburg is the hottest European link to the US attacks, but Italy is also involved.
"A trail runs to Italy in connection with some stolen pilot uniforms," said Europol spokesman Juergen Storbeck.
The six people arrested on Friday in Belgium and the Netherlands have been described as Islamic militants suspected of planning attacks on US targets - but no immediate link has been made with the US hijackings.
Two Uzi machine pistols were found during a house search in a wealthy suburb of Brussels, where two of the six were seized.
French media report that one of them - said to be a man of North African origin about 30 years old - was planning to attack targets in France, including the US embassy in Paris.
Belgian police say they are examining a possible connection between one of the people held in Belgium and Osama Bin Laden.
French examining magistrates who have opened an inquiry into possible links between Islamic militants in France and the gang behind the US attacks
They are expected to travel to Belgium on Monday.
Swiss Defence Minister Samuel Schmid said on Friday that other attacks had been planned in the United States and that the trail of possible perpetrators led to Switzerland, among other European countries.
He said Swiss authorities were working closely with the United States, but Europol's Juergen Storbeck called on Saturday for better co-operation with American counterparts.
He said Europol was only receiving "very scarce information" from the US.