The investigation and the evidence

If the US is to maintain international backing for its war on terrorism, the strength of evidence linking Osama Bin Laden and his al-Qaeda organisation to the attacks is crucial. Much of this evidence is not yet in the public domain. BBC News Online looks at the investigation to date and considers the information that has emerged.

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The investigation:

Within hours of the attacks, the Federal Bureau of Investigation launched what has become the biggest manhunt and investigation in US history.

Hijacking suspects

Flight 175: Marwan al-Shehhi, Fayez Ahmed, Mohald al-Shehri, Hamza al-Ghamdi and Ahmed al-Ghamdi

Flight 11: Waleed M al-Shehri, Wail al-Shehri, Mohammed Atta, Abdulaziz al-Omari and Satam al-Suqami

Flight 77: Khalid al-Midhar, Majed Moqed, Nawaq al-Hamzi, Salem al-Hamzi and Hani Hanjour

Flight 93: Ahmed al-Haznawi, Ahmed al-Nami, Ziad Jarrahi and Saeed al-Ghamdi

More than 4,000 FBI agents are involved, with 3,000 support staff and more than 400 laboratory personnel.

On 14 September, the FBI released the names of the 19 hijackers it believes carried out the attacks. There is some doubt about four of the people named as some of the hijackers may have been travelling on false documents.

Several of those arrested are reported to have had connections to the hijackers or Osama Bin Laden, and some may have been planning other hijackings.

This is a murky area in which unattributed briefings and misinformation must be taken into account.

Click here for pictures and more details on the alleged hijackers.

Official FBI briefings have been short on detail, though many US newspapers have carried unverifiable reports sourced to unnamed security or intelligence officials.

Here are some of the important developments in the US investigation:

• More than 1,000 people have been arrested in the US in connection with the 11 September attacks according to the Justice Department - the majority for immigration violations
• US civil liberties groups have criticised the government for failing to release information about the individuals arrested in connection with the events of 11 September
• US investigators were reported to have found a hire car at Boston airport containing a copy of the Koran and an instruction manual on how to fly a plane
• The passport of one of the hijackers was reportedly found in the wreckage of the World Trade Center
• Four men were detained in Texas, alleged to have been carrying thousands of US dollars and knives similar to those used by the hijackers
• Three men were arrested in Detroit, allegedly in possession of airport plans
• Homes and hotels where the attackers are believed to have stayed were raided, and flying schools where they may have trained were searched.

People suspected of having links to Bin Laden and al-Qaeda were detained across the world. Arrests were made in Britain, France, Germany, Spain, Belgium, the Netherlands, Malaysia and Thailand.

• Djamel Beghal, a French Algerian, was extradited to France from the UAE in connection with the alleged plot
• Another French-Algerian, Kamel Daoudi, was arrested in Leicester in the UK and extradited to France
• In Thailand police searched 15 Arab men wanted by the CIA in connection with the attacks
• In Britain, Lotfi Raissi was arrested. He is alleged to have given flight training to four of the hijackers
• Three hijack suspects were tracked back to Hamburg in northern Germany where they are alleged to have attended university
• Belgian police arrested two men, alleged to be Islamic militants, and seized a huge store of chemicals in their Brussels flat
• In Rotterdam Dutch police arrested Islamic militants suspected of some connection to the attacks
• Six Algerians were arrested in Spain on suspicion of links with Bin Laden.

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The trail to Bin Laden:

Money transfers: US investigators are reported to have established a direct link between Mohammed Atta, the man they allege led the hijackers, and Bin Laden's al-Qaeda.

Four of the alleged hijackers of the first plane to strike the World Trade Center

They say that they have evidence showing money transfers from an account held in the United Arab Emirates by a leading Bin Laden operative, Mostafa Mohammed Ahmad, and an account in the name of Atta at a bank in Florida. These are said to have taken place on 8 and 9 September 2001. Atta is further alleged to have returned unused funds to the same bank account in the UAE.

Egyptian Islamic Jihad: Atta is also said to be a member of Egyptian Islamic Jihad, the group led by Ayman al-Zawahri - a man believed to be a close associate of Bin Laden and to have a leading role in al-Qaeda.

Links to al-Qaeda: Two other alleged hijackers, Khaled al-Midhar and Nawaq al-Hamzi are said to have been filmed at a meeting in Kuala Lumpur in Malaysia with other known al-Qaeda operatives.

The attacks launched the biggest investigation in FBI history

An official British document outlining the case against Bin Laden alleges that one of the Saudi-born militant's closest and most senior associates planned the 11 September attacks. This associate, believed to be a senior al-Qaeda leader, is not named.

US officials say that most of the alleged hijackers trained at al-Qaeda bases in Afghanistan.

Intercepts: US officials have said that they intercepted communications by Bin Laden in the days before 11 September which indicated that a big operation was imminent.

The comments, on a satellite phone which Bin Laden must have known was being monitored, may have been intended to confuse American intelligence services. The intercepts are alleged to have hinted at an attack against American targets outside the US.

German intelligence monitored a phone call by a man they suspected of having linked to al-Qaeda. He is alleged to have said: "We have hit the targets."

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Strength of the evidence:

There is no direct evidence in the public domain linking Osama Bin Laden to the 11 September attacks.

At best the evidence is circumstantial.

Of this, perhaps the strongest leads are the alleged financial transfers between an al-Qaeda operative and the man alleged to have led the hijackers.

Other evidence - the intercepts, Mohammed Atta's link to Egyptian Islamic Jihad, the ties of other hijackers to al-Qaeda - is even less firm.

The evidence is not being judged in a court of law. It only needs to persuade governments around the world to back the US-led war on terrorism and to a lesser extent to carry public opinion.

US and British officials have indicated that they are unable to reveal all the evidence for security reasons.

When asserting that Bin Laden is behind the attacks, US and UK officials lean heavily on what they believe to be Bin Laden's record and his connection to other terrorist attacks.

They are in effect arguing that the attacks are part of a clearly discernable pattern linked to previous attacks - notably the bombings of the USS Cole in Yemen in October 2000, and two US embassies in East Africa in August 1998.