New York's subways were under increased security yesterday after receiving what local officials described as the most specific threat to date of a terrorist attack in the coming days.
"This is the first time that we have had a threat with this level of specificity," Mayor Michael Bloomberg said on Thursday. "It was more specific as to target, it was more specific as to timing."
Although the intelligence behind the alert had yet to be corroborated, Police Commissioner Ray Kelly said that it had been deemed of "sufficient concern" to enhance counter-terrorism operations on the subway network, as well as buses and ferries.
Officials refused to provide details of the precise nature of the threat, although Kelly said an existing policy of random searches on the subway would be intensified, with a particular emphasis on baggage and baby strollers.
Despite the increased security, subways were running as normal and New York City remained on the second highest alert level, where it has been since the Sept. 11, 2001, attacks.
US media quoted Department of Homeland Security officials as saying they doubted the credibility of the threat.
"The intelligence community has concluded that this information is of doubtful credibility," the department said in a statement.
CNN quoted a senior Pentagon official as saying US troops, acting on the same intelligence, had taken part in a raid south of Baghdad late Wednesday and rounded up several al-Qaeda operatives.
The head of the FBI's New York field office, Mark Mershon, said the new threat had been "partially disrupted" but declined to elaborate beyond confirming that no arrests had been made in the city itself.
Bloomberg said the warning of the attack had come several days ago, but a public announcement was delayed.
"There were operations taking place that we thought were in the interest of ending the threat, and to release the information earlier could have jeopardized the lives of those conducting those operations," he said.
The new warning came just hours after a major speech on terrorism by US President George W. Bush in which he sought to convince Americans that Iraq was a central front of the anti-terror campaign.
Bush said the US had foiled three al-Qaeda terror strikes on its soil since the Sept. 11 attacks, and stopped terror groups casing US targets and infiltrating operatives into the country.
"Evil men obsessed with ambition and unburdened by conscience must be taken very seriously and we must stop them before their crimes multiply," Bush said.
In the immediate wake of the July suicide bombings on the London transport system that claimed 52 lives, New York flooded its subway network with police patrols and introduced a policy of random bag searches.
The New York subway -- fourth largest in the world in passenger volume -- carries 4.5 million passengers on an average working day.
"We've never had before a specific threat to our subway system," Bloomberg said.
"There have been people all the time on the Internet and every place else, saying you know, `I'm going to go get those guys,'" he said. "But suffice it to say [this time] its importance was enhanced above the normal level, by the detail that was available to us," he said.
In a city that still bears the physical and psychological scars of the Sept. 11 attack four years ago, the presence of police, anti-terrorist units and National Guard troopers has become a common feature of subway travel.