Several people who had driven into the centre's underground car park just before the explosion were being questioned by police, and a Reuter report from Washington said that experts were examining intercepted global communications for any sign of a foreign hand in the blast, in which five people were killed and more than 1,000 injured.
There were conflicting reports from unofficial police sources on the possible contents of the bomb. One report said that it might have consisted of between 500lb and 1,500lb of dynamite of the kind used on building sites; another, that it was probably made from a mixture of fuel oil and fertiliser.
Because of the size of the explosion, investigators believe that the bomb was carried into the car park in more than one vehicle. Some of the witnesses already interviewed by police said that they saw several vehicles moving suspiciously near the trade centre just before the bomb exploded.
It was normal practice in the car park for the licence plates of cars to be recorded, and the list has provided investigators with a leads after the car numbers were compared with computerised intelligence information.
Those sources putting forward the dynamite theory said that it seemed to indicate that the bombers were not used to dealing with explosives because of the large quantities they needed to create a huge explosion.
Terrorist bombs are usually made of plastic explosive. The dynamite theory started when traces of nitrate, a component of dynamite, were discovered in the debris. However, this does not exclude the theory that it was a fertiliser mixture, since this also could have contained nitrates.
The trade centre buildings yesterday were still closed for business, except to commodity traders who would have faced losses of millions of dollars if they had been denied entry.
Since Friday's explosion there have been 200 bomb threats to targets in Manhattan. Normally there are no more than 10 a day.Reuse content