The trail to Bin Laden

By BBC News Online's Richard Allen Greene

Saudi-born militant Osama Bin Laden emerged as the top suspect in the terror attacks on New York and Washington within hours of the events of 11 September.

US President George W Bush and UK Prime Minister Tony Blair have both said they are convinced of his involvement.

Mr Bush is convinced Bin Laden was involved

Mr Bush said on 19 September that when the US acts against the people it thinks were behind the attacks, "we will take action because we believe - because we know we'll be in the right".

Mr Blair, for his part, said "it's fairly clear where the evidence is tending".

Other countries, however - including some of Washington's European allies - have said they want proof before they are willing to go after Bin Laden.

But, despite a massive worldwide investigation, evidence that would stand up in a court of law may be very hard to come by.

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Investigators will rely on a number of methods to link Bin Laden to the deadly attacks:

• Tracing the people who carried out or helped with the hijackings back to Bin Laden's al-Qaeda organisation, which is based in Afghanistan

• Intercepting communications between Bin Laden and his suspected operatives around the world

• Showing that Bin Laden's money financed the attacks.

Bin Laden's own words are also likely to count against him - he has made clear his determination to strike against the US.

None of these methods is without its limitations.

Identifying associates

The FBI announced the names of 19 people they say carried out the four hijackings soon after they took place.

Hijacking suspects

Flight 175: Marwan Al-Shehhi, Fayez Ahmed, Mohald Alshehri, Hamza Alghamdi and Ahmed Alghamdi

Flight 11: Waleed M Alshehri, Wail Alshehri, Mohamed Atta, Abdulaziz Alomari and Satam Al Suqami

Flight 77: Khalid Al-Midhar, Majed Moqed, Nawaq Alhamzi, Salem Alhamzi and Hani Hanjour

Flight 93: Ahmed Alhaznawi, Ahmed Alnami, Ziad Jarrahi and Saeed Alghamdi

They are now devoting huge resources to tracing their movements in the run-up to the attack, including trying to find out who paid for their flight training courses, rent and so on.

But there are now suspicions that the hijackers were travelling under false names and with forged documents.

"We may never know their real identities, which will stymie the investigation," one British security expert who asked not to be named told BBC News Online.

Hundreds of people have been detained in the US, mostly as material witnesses or due to immigration irregularities.

And on 19 September, the US made its first arrests, of three men in Detroit, who were held on charges of possessing false documents, including US immigration forms and a visa.

Agents have since detained a fourth man, Nabil al-Marabh, a search of whose home turned up a diagram of Detroit's airport.

But the very nature of the charges against the three men arrested on Wednesday - having false papers - indicates how hard it will be to establish their links with Bin Laden. Initial reports said the authorities had not even established their nationalities.

Intercepts

The US places a great deal of faith in hi-tech surveillance methods, but they too have their drawbacks.

There are doubts about some hijackers' identities

A recent FBI call for help from people able to translate from Middle Eastern and South Asian languages - Arabic, Pashtu and Farsi - highlights a limitation of intercepting communications: Intelligence services must be able to understand the messages they are intercepting.

Even if sufficient translators are found, the elaborate network of operatives who carried out the attack will almost certainly have communicated in code.

The British security expert said militant organisations have been known to exchange messages by placing innocuous-looking advertisements on web sites, for example.

"Unless you're at one end or the other, you'll never know what the message is - there are millions of messages exchanged on the internet, and millions of web sites," he said.

The militant cells "use the basic techniques of espionage. One shouldn't underestimate their sophistication", he added.

Money laundering

Financial trails may be equally difficult to unravel.

Tracking down money that has been laundered may at best be time-consuming - at worst, impossible.

John Ashcroft: Foreign states involved

And some financial experts say that networks such as Bin Laden's may not transfer money through the international banking system at all.

Instead, they may use a system known in Pakistan and Afghanistan as hundi.

The trust-based system involves giving money to respected brokers in one country whose associates give comparable sums to recipients in another. Regular settling of accounts between various brokers may never even be recorded, making such transactions virtually untraceable.

Modus operandi

In the end, the evidence that people may find most convincing are Bin Laden's own statements and record.

The networks that conduct these kinds of events are harboured, supported, sustained and protected by a variety of foreign governments

John Ashcroft,
US Attorney General

He has denied involvement in the World Trade Center and Pentagon attacks, but he has for years been calling for a global war against the United States and against Jews.

He is suspected of involvement in a 1993 attack on the World Trade Center and a 2000 attack on a US warship, the USS Cole.

In 1998, he issued what amounted to a declaration of war against the US.

The people convicted of the bombings of US embassies in Kenya and Tanzania that year named him as a backer - a charge he denies.

Investigators will certainly have noted the similarity between one element of the east African bombings and the attacks on New York and Washington - carefully co-ordinated simultaneous attacks on targets far away from each other.

But even those who place the blame on Bin Laden are now saying he was probably not solely responsible.

"It is pretty clear that the networks that conduct these kinds of events are harboured, supported, sustained and protected by a variety of foreign governments," US Attorney General John Ashcroft said on Wednesday.

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