'Working together, we'll find out who was involved and why this happened,' Mr Clinton said. Rescue workers were yesterday searching for two people believed to be buried by rubble beneath the world's second-tallest building by the blast, which killed five and injured 1,042 people.
Investigators from the FBI and New York Police Department said that they had found explosive residues, including traces of nitrate, near the centre of the blast, but they had not determined who might have planted a bomb.
Police believe a very large car bomb was left in an underground car park directly beneath the World Trade Centre, whose twin 110-storey towers dominate lower Manhattan. It exploded at 12.18pm on Friday, ripping holes through five underground floors, instantly killing at least four port authority workers nearby and trapping some 50,000 people in the building above.
Although 19 calls were received claiming responsibility - including some from self-styled Serbian and Croatian groups - the New York police chief, Raymond Kelly, said none had telephoned before the blast. The first was received more than an hour later. A bomb defused yesterday outside the US embassy in Zagreb fuelled speculation about a connection with former Yugoslavia.
From the outside the towers, built in the late 1960s, and after Chicago's Sears Tower the world's tallest buildings, looked undamaged apart from some broken windows. But in the underground car parks and a railway station on a line to New Jersey the explosion left a jagged hole 100ft by 200ft in five levels of reinforced concrete.
Until Friday, the US had remained largely invulnerable to politically-inspired bombings at home. The most serious attacks against US citizens and interests, notably the death of 242 US Marines in Beirut in 1983 and the destruction of the Pan-Am airliner over Lockerbie in 1988, have occurred abroad. Although the President and White House are heavily protected, security around prominent buildings is slack or non-existent.
All but one of the World Trade Centre's seven buildings will be closed tomorrow, leaving 50,000 workers and some of the leading US financial houses and commodity exchanges without a place to do business.
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