LONDON, England -- Police across Europe have been put on alert as they step up security following the terror attacks in the United States.
In a joint operation in Belgium and the Netherlands, police arrested six suspected Islamic extremists, authorities said on Friday, according to The Associated Press.
Two of the men were arrested on Thursday in Brussels. A spokeswoman for the Brussels Public Prosecutor's Office told Reuters two Uzi machine pistols were found during a house search.
The other four were arrested in the Dutch port city of Rotterdam on Thursday night.
However, Dutch prosecutor's spokesman Wim de Bruijn said there was no connection with Tuesday's attacks. Dutch television reported that they were possibly members of an international network of radical Muslims.
In Germany, authorities in Hamburg acting on tips from the FBI detained an airport employee and also questioned a woman on Thursday, but found no reason to hold either.
The man, whose name was not released, was arrested after investigators searched apartments in the city looking for evidence linked to the attacks.
Italian police in Rome re-opened the case of a theft of uniforms and badges belonging to two American Airlines pilots in April, while in France, special anti-terrorism prosecutors investigated links with militant Islamic networks in their country.
In England, a man in his 40s was being quizzed under the Prevention of Terrorism Act after he was arrested by police at Heathrow Airport on Thursday afternoon. Police would not say if the arrest was linked to the U.S. attacks. He was later released without charge.
Germany's top prosecutor revealed on Thursday that three men of Arab descent who died in the crashes are believed to have lived in the northern German city of Hamburg.
Two of the men have been identified as Mohamed Atta, 33, and Marwan Alshehhi, 23, both from the United Arab Emirates.
Both had earlier been named as former students of a Florida flight school and are suspected of having flown two of the hijacked jets. Up until last year, both men had been studying at Hamburg Technical University.
They had been registered for several years as residents of Hamburg, prompting police to search an apartment where they had lived. However, it had been unoccupied since February, and recently renovated, making the search for evidence difficult.
University president Christian Nedess told AP Alshehhi had studied one year at the university, and Atta had studied for eight years, completing one degree and starting a second.
He declined to say what the men had studied, but said it would not have given them technical knowledge that would have aided in the terror attack.
According to Hamburg authorities, Alshehhi left Germany for the U.S. on May 2 2001, although it was unclear when Atta had left. Both had travelled to the U.S. previously.
Sources have told CNN that a car impounded at Boston's Logan airport was rented by Atta. The vehicle contained materials, including flight manuals, written in Arabic. An apartment in Coral Springs, Florida, linked to Atta was also searched.
Quoting Nedess, AP said Atta was aboard the plane that crashed into World Trade Center Tower One, while Alshehhi was on the jet that struck the other tower 20 minutes later.
Other students at the university in Hamburg are also understood to be under investigation.
The prosecutor's office, which has put more than 60 people on the investigation, said at present no evidence had been found linking the Hamburg suspects to Osama bin Laden, the Saudi millionaire accused by the United States of sponsoring terrorists.
Chief federal prosecutor Kay Nehm told reporters that investigators believed the men, and a third suspect who also died, belonged to a terror group formed "with the aim of carrying out serious crimes together with other Islamic fundamentalist groups abroad, to attack the U.S. in a spectacular way through the destruction of symbolic buildings."
In addition to the man arrested in Hamburg, German police are searching for a man with an Arab background. Nehm said he was suspected of murder and air piracy as well as membership in a terrorist organisation, but gave no further details.
In Italy, the terror attacks in New York and Washington have prompted police to take another look at the robbery of uniforms and documents belonging to two American Airlines pilots.
Colonel Gianfranco Cavallo of the Italian Carabinieri paramilitary police said the thieves broke into four rooms at the Hotel Nazionale in central Rome on April 6 2001. Two of the rooms were occupied by the pilots.
Reuters reported that one pilot said the thieves made off with the room's safety box, which held his passport, an access badge to American Airlines offices in airports worldwide, his driving licence, wallet and keys.
The other pilot lost his passport, the jacket and tie of his uniform and a camera.
Cavallo said the pilots told police they had reported the incident to their company.
Meanwhile, a French radio station reported on Thursday that the arrest of an Islamic militant in Boston last month by the FBI had been linked to Osama bin Laden by French intelligence reports, but that the U.S. apparently did not act on them.
Europe 1 radio reported that U.S. police had arrested a man with dual French and Algerian nationality who had several passports, technical information on Boeing aircraft and flight manuals. The man had been taking flying lessons, it added.
Reuters said that at that time, French security services provided information clearly identifying him as an Islamic militant working with Osama bin Laden, the Afghan-based Saudi millionaire accused of masterminding the U.S. attacks.
"He has a pedigree as long as your arm, an investigator said," Europe 1 radio quoted by Reuters reported. "He belongs to the Pakistani-Afghan network that trains Osama bin Laden's soldiers."
It said the man was in jail, but had refused to co-operate with investigators.
"He has the profile of someone who could prepare or lead terrorist operations," it said.
"This information was transmitted by French security services to the FBI but apparently got lost in the enormous American police machinery," it added.
"The inquiry that might have been able to avoid everything was not started. There was no special alert transmitted to airport authorities in the U.S.," Europe 1 radio concluded. According to Reuters, a French judicial source said the U.S. had now requested information on the militant.