Officials hunting the terrorist mastermind behind the horrific attacks on New York and Washington were building a clearer picture yesterday of how the devastating operation was carried out.

More than 4,000 FBI agents have been assigned to work on the case, chasing numerous leads internationally for details of the plot and the terrorists behind it.

Agents have established that up to 50 people were involved in the attack on the World Trade Centre and the Pentagon, with the blame increasingly being pointed at terrorist leader Osama bin Laden. A total of 18 hijackers were on the planes.

Last night, Secretary of State Colin Powell identified bin Laden as the prime suspect. He said he would press the president of Pakistan for information on his operations.

While the focus of the investigation concentrates on America's east coast, police across the Atlantic in Italy and Germany were also following possible leads.

Italian police announced they were reopening an investigation into the theft of uniforms and ID badges belonging to two American Airlines pilots, one of whom was born in Dublin.

Colonel Gianfranco Cavallo, of the Italian Carabinieri paramilitary police, said: "We are re-examining the robbery in light of what has happened."

He said thieves broke into four rooms at the Hotel Nazionale in central Rome in April.

One pilot lost 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.

One of the pilots was a 39-year-old with dual Irish-American citizenship.

The announcement came as German police said they had arrested one man in the northern port city of Hamburg in connection with the investigation.

It was believed the man was a colleague of two suspected hijack suicide pilots - Mohamed Atta and Marwan Alshehhi - who crashed into the World Trade Centre.

The arrest came after a search of a flat where it is believed one of four cells thought to be responsible for the attacks was formed early this year.

Hamburg police chief Gerhard Mueller said the arrested man was detained pending an investigation, but refused to give details about his age and nationality.

He said later the man had a job "connected to air transport" and had worked at an airport, but refused to give further details.

Atta was on the passenger list of American Airlines Flight 11 and Alshehhi was on board United Airlines Flight 175 - the two planes that were hijacked after take-off from Boston and crashed into the WTC's twin towers.

Both apparently entered the US from Germany and had been students at a technical university in Hamburg.

Earlier, the FBI issued photos of Atta, 33, and Alshehhi, 23 who had studied at Huffman Aviation School and stayed in Venice, Florida.

Meanwhile, agents in the US have identified about 40 infiltrators who had a hand in the attacks. Ten of those are thought to be at large, and the remainder either died in the attacks or are "accounted for", the LA Times reported, giving no further details of their status.

FBI agents worked to track down the men, with the massive investigation stretching from the Canadian border, where they suspect some of the hijackers entered the country, to Florida, where some of the participants are believed to have learned how to fly commercial jets before the attacks.

In total 27 terrorists are thought to have been trained to fly, the LA Times said.