They plotted and played. But who were they?

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Unlike terrorists in other incidents, these were not run-of-the-mill, young men ordered to make a quick hit and then disappear. They had been to university, attended flying schools, lived in neighbourhoods and led normal lives – lives that left invoices, receipts, ticket stubs and driving licences behind. Some of them even gambled and went to shows in Las Vegas.

The investigators also say the terrorist attacks were part of an expensive operation costing $200,000 (£137,000) compared with $20,000-$40,000 for the 1993 attempt to bomb the World Trade Centre. And the hijackers left a maze of credit card payments, car rentals and hotel bookings.

They appear to have made a genuine "breakthrough" with the arrest of Nabil Almarabh in Chicago. Mr Almarabh was at the centre of a long-running customs probe into Mr bin Laden's links with the US. Based in Boston, scene of two of the hijackings and the home of a likely bin Laden cell, he had two associates, Ahmed Aghamdi and Satam Al Suqami. The trio were involved in deals that resulted in cash flowing from banks in Boston to accounts overseas, thought to be controlled by Mr bin Laden's Al-Qa'eda group.

Mr Almarabh was targeted because he was a close friend of Raed Hijazi, a fellow taxi driver, convicted in absentia in Jordan of planning bombings on US tourists during the millennium celebrations.

Mr Hijazi was born in Los Angeles to Palestinian parents, grew up in Jordan and studied business at California State University, Sacramento, before moving east to Boston. He applied for a taxi permit and listed Mr Almarabh as a person to contact in an emergency. Another taxi driver friend of Mr Hijazi's, Bassam Kanji, died last year in a Lebanon gunfight between Islamic militants and the Lebanese army. Lebanese papers named him as a bin Laden supporter.

Last year, Mr Almarabh was put on probation over a stabbing incident in Boston. He broke his terms and disappeared. The customs investigation went cold. It was only when customs officers recognised the names of two of the hijackers, Ahmed Alghamdi and Satam Al Suqami, that alarm bells began to ring and an all-points alert was issued. Mr Almarabh was picked up while working as a clerk in a liquor store in a Chicago suburb.

To the anxiety of the authorities, they discovered his journey from Boston to Chicago took in Michigan, where he obtained a driving licence to carry hazardous and toxic materials. There is no evidence he ever worked for a road haulage firm specialising in hazardous and toxic materials.

Mr Almarabh may have been in on the hijackings. So too, might an older man seen by witnesses with some of the hijackers in the months before the attacks. The FBI is frantic to find this man, after witnesses in two states gave the same description to investigators. Unfortunately, the agents do not have much to go on: he has "salt and pepper hair".

They suspect his presence mirrored other bin Laden attacks where a senior aide arrived to instruct the bombers. At the time, there was nothing sinister about a grey-haired man visiting some respectable, younger Arab friends. Now, the agents need to find him.