The biggest manhunt in US history gathered pace yesterday, after FBI agents said they had identified up to 50 people responsible for Tuesday's suicide plane attacks on New York and Washington. From Florida to the Canadian border, about 4,000 FBI agents - a quarter of the agency's investigators - were searching for 10 suspects believed to have helped carry out the devastating attacks. They know the whereabouts or fate of another 40. No arrests have been made but several people are under interrogation after raids in Florida and Boston. FBI agents were last night questioning a man they believe could be connected to the attacks as a strong Florida connection to them emerged. Adnan Bukhari was picked up at a house in Venice, Florida, and it is understood he trained at the same pilot school as Mohammed Atta, whom investigators say was one of the hijackers who died on one of the flights which hit the World Trade Centre. Officers also raided the house next door to Bukhari's, but a man they were looking for was not there. Bukhari's name was connected to a suspicious car - containing a flight manual in Arabic - which police found at Boston's Logan Airport on Wednesday, from where two of the flights were hijacked. His brother, Ameer, who was also said to have trained as a pilot at Huffman Aviation in Venice, died in a plane crash last year, investigators said. Attorney-General John Ashcroft said evidence had emerged of a network with 'significant ground support' across the US. He said there could have been as many as six hijackers on each plane. The hunt spread to Germany, where police arrested one man in Hamburg during a search of eight apartments, including one used by Atta. Taleban leaders in Afghanistan were yesterday reported to have placed Saudi terrorist mastermind Osama bin Laden under house arrest. US sources suspect he is the only person with the resources and connections necessary for the highly co-ordinated attacks. The Islamic regime's envoy in Pakistan said it would 'study' any request for his extradition, as Afghans braced for massive US retaliation for the world's worst terrorist atrocity. The reports of bin Laden's arrest, in Pakistani and Middle Eastern media, were dismissed by Taleban sources as 'concocted', and were greeted with suspicion in Washington. 'We are aware of the risk of false reports geared to limiting any military response,' a source in the administration of President George W. Bush said. The New York Times claimed the hijackers were all followers of bin Laden. The paper, citing government sources, said at least two known bin Laden associates had slipped into the US before immigration authorities could be alerted - the first concrete sign of a serious intelligence breakdown. The Government has yet to officially confirm that bin Laden is its chief suspect, but sources claim Mr Bush was informed he was the target of investigations. As the White House orchestrated an international drive to build support for both the investigation and possible military retaliation, spokesmen for Mr Bush claimed he was a direct target for assassination in the attacks. Press Secretary Ari Fleischer said there was 'real and credible' information that the White House and Air Force One - the presidential plane - were targets. Other officials said threats had been phoned through to the Secret Service displaying knowledge of security plans. While Mr Bush flew to secret locations across the country, Vice-President Dick Cheney led rescue and military efforts. Early this morning it was reported that five New York firefighters had been pulled out alive, probably from the debris under the trade centre's north tower. The firefighters had been trapped in their fire vehicle for two days, the reports said. The FBI arrested a man yesterday after a telephoned bomb threat prompted a temporary evacuation of rescue teams from an area of the Pentagon shattered by the terrorist attack. Defence Department spokesman Bryan Whitman confirmed that a telephone threat had been received and that dozens of rescue and medical workers were evacuated for about two hours. Mr Bush declared today a day of 'prayer and remembrance', the White House said. America's military remained on full alert, with its most sensitive installations under intense security and its air space protected by a network of ships and submarines off both coasts. At the same time, signs of normality were returning after days of confusion and chaos. The nation's 450 commercial airports opened at 11pm last night under intensified security that could seriously slow air travel. New York's stock exchange will remain closed until today at the earliest. The three-day shutdown is unprecedented since the Great Depression. Chief Secretary Donald Tsang Yam-kuen, meanwhle, will be one of the first foreign public servants to meet Secretary of State Colin Powell since the attacks. A meeting organised before Tuesday is still likely to go ahead, State Department officials said.