Heavily armed police and FBI agents swarmed a downtown hotel in Boston in what appeared to be a search following Tuesday's assault on the World Trade Centre.
Separate FBI activity was reported in southern Florida, meanwhile, where agents searched homes and businesses, apparently paying closest attention to an aviation school where two suspects may have received flight training.
Boston, whose Logan Airport was the scene of two of the four hijackings, has become central to the investigation. Five Arab men who boarded the Boston planes were identified as suspects. At least two were brothers travelling on passports from the United Arab Emirates and one was understood to have trained as a pilot.
Two of the men had flown to Logan from Portland, Maine, reportedly after crossing the border from Canada. However, Canada's prime minister, Jean Chrétien, said that there was no confirmation of this.
The bags of one of the men did not make the connection. According to local reports, they contained a copy of the Koran, an instructional video on flying commercial aircraft and a fuel consumption calculator.
The other hijackers were thought to have arrived at Logan by car. A rented Mitsubishi Mirage in the airport was impounded by the FBI, which did not deny claims that it contained Arabic flight manuals. An airline crew tag was also found. Earlier reports said the car was from Canada, but there were also claims it was local. Heavily armed agents swooped on the Westin Hotel in Boston. One man was reported arrested and another wounded. Later, officers also raided the Park Inn outside the city.
Elsewhere, investigators launched what was being billed as the biggest manhunt in US history. "Think of it as several Oklahoma City bombings wrapped up in one," said the former FBI deputy director, William Esposito. An all-points alert was issued for two vehicles thought to be tied to the hijackings. The FBI said it wanted to trace three men in a white Chevrolet van with a New Jersey plate and "Urban Moving Systems" on its side, and a blue-green Toyota with two Arabs inside which had been asking for airport directions. In New Jersey, three people were taken into custody for questioning after police stopped a van near the George Washington Bridge.
In Florida, the FBI was targeting two suspected hijackers, one of whom was believed to be a known supporter of Osama bin Laden, the Saudi dissident widely thought to be behind the attacks.
A local man and his wife interviewed by the FBI said agents told him that two men who stayed in his home for a week in July 2000 were the hijackers. The couple accepted the two men, who were training at Huffman Aviation, a local flight school that employed Mr Voss, as a favour to the company. Charlie Voss said that the agents identified the men as Mohamed Atta and Marwan.
One of the men was reportedly identified from the manifest of one of the hijacked planes. The bureau obtained warrants to search properties in Broward County and Daytona Beach.
The authorities also were developing intelligence linking the suspected attackers to a band of bin Laden sympathisers in Canada, some of Algerian origin, who are suspected of planning an unsuccessful terrorist attack on US soil at Los Angeles airport during the millennium celebrations.
The hunt was being co-ordinated by John Ashcroft, the US Attorney General, and senior law enforcement officers from a fifth-floor Special Emergency Operations Centre at FBI headquarters. At one stage they were joined by US Army Special Forces officers dressed in camouflage gear.
Officials were anxious to be seen to be on top of the situation, ordering thousands of agents from the FBI, CIA and other US government organisations at home and abroad to find the perpetrators. This was as much a face-saving exercise, as they sought to refute claims that they should have been more prepared against a large-scale terrorist onslaught on home soil, as a concerted criminal investigation likely to yield immediate results.
President Bush has spent much time and effort to impress upon the American people the dangers of "asymmetric" threats to the US following the end of the Cold War. These have ranged from attacks on US personnel overseas to nuclear missiles launched from a rogue state to poison gas being released on subways.
To that end, the counterterrorism budget has doubled from $6bn in 1995 to $12bn this year. In addition, Mr Bush intends to spend $8.3bn next year to begin work on his "star wars" defence programme against nuclear attack.
But, while Americans were braced for nuclear missiles coming from Iraq or North Korea they were not alerted to the possibility of suicide missions aimed at their symbols of financial and military power.
Crime scene inquiries were established in New York, Washington, Boston, Newark in New Jersey (where the plane that crashed outside Pittsburgh was seized) and at Pittsburgh. FBI disaster response and evidence recovery teams were rushed to the crash sites, but their efforts were hampered in New York and Washington by the protocol ordering them to wait until all survivors are released.
They were able to study CCTV footage from the airports, passenger lists and tapes of phone calls from known bin Laden sympathisers. They were optimistic about recovering the black box voice recorder from the Pittsburgh aircraft. Hopefully, they said, that might provide some leads as to the hijackers' identities. The recorders in the New York and Washington DC disasters were somewhere under mounds of rubble and could take days, if not weeks to find, if they have survived the carnage at all.
At the CIA headquarters in Langley, Virginia, analysts were poring over intelligence material and telephone intercepts gathered by the National Security Agency. Senator Orrin Hatch, a Republican, said after receiving a briefing from officials, that electronic eavesdropping had picked up bin Laden associates discussing the success of the operation. "We have information that indicates representatives that are affiliated with Osama bin Laden were actually saying over the airwaves, private airwaves that they had hit two targets," said Mr Hatch.
However, officials were counselling caution. All the hijackers had been killed and the sort of painstaking search for evidence that occurred in the Lockerbie bombing, where they were able to piece together the Pan Am aircraft, was unlikely in New York and Washington, given the scale of the damage. It was probable, they said, that the terrorists had comprised a tight cell, in which case, proving that their leader was bin Laden, for example, might also be impossible.
Officials stressed that they were dealing with around a thousand instant leads in an investigation into what was a sophisticated and presumably long-planned onslaught. In a fevered atmosphere, they were having to cope with thousands of Americans volunteering information, much of it useless.
A government website inviting tips from the public was jammed within minutes of being opened. Toll-free numbers for people with leads were also deluged.
Officials were examining previous bin Laden-inspired incidents, including the 1998 bombings of the US embassies in Nairobi and Dar es Salaam, and a failed plot to attack Los Angeles airport. The fact that Tuesday's attacks came just one day before a bin Laden associate was due to be sentenced in New York for his role in the Dar es Salaam bombing was not lost on the authorities.
Boston features in the bin Laden files already, as the home of at least two of his agents. Raed Hijazi, a Boston taxi driver, was sentenced to death in Jordan last year for plotting terrorist atrocities. His friend and fellow taxi driver Bassam Kanj was killed in Lebanon last year leading a militant attack.Reuse content