The World Trade Center attack devastated New York
After the suicide attacks on the World Trade Center and Pentagon which killed about 3,000 people, the United States launched the biggest criminal investigation in history.
The hijackers were quickly identified. It emerged that they had taken flying lessons in Florida in preparation for steering the passenger planes into their targets.
Further inquiries by the US authorities linked them to the radical Islamic militant group al-Qaeda which was set up by Osama Bin Laden - the Saudi-born dissident who was already blamed by the US for attacking its embassies in East Africa in 1998 and the warship USS Cole in 2000.
The US launched a military offensive in Afghanistan in an effort to track down Bin Laden. It succeeded in shutting down al-Qaeda training camps within the country and toppling the Taleban rulers of Afghanistan, but has so far failed to find Bin Laden.
The US and its coalition allies have also vowed to eliminate the network of al-Qaeda cells thought to have been established throughout the Western world.
Dozens of al-Qaeda and Taleban suspects captured in Afghanistan and elsewhere have been imprisoned in an American military base at Guantanamo Bay in Cuba.
International co-operation has led to hundreds of arrests in more than 50 countries, with several nations changing their laws to give them wider powers to apprehend alleged activists.
In the US more than 1,100 people have been detained, many from Middle East countries, and charged with immigration offences.
Who is investigating?
The Federal Bureau of Investigation (FBI) is at the centre of the investigation. Their results have not been disclosed.
Intelligence agents have reportedly foiled a number of planned attacks since 11 September
Both the FBI and the Central Intelligence Agency (CIA) have been criticised for failing to share and analyse information properly. Congress accused them of not "joining up the dots" before 11 September.
The Senate and House Intelligence oversight Committees are now investigating what went wrong.
Since the attacks, Attorney General John Ashcroft and FBI Director Robert Mueller have refocused the bureau's efforts on preventing future terror attacks.
The powers of the FBI to detain people without charges have been widened in a move that has been criticised by human rights groups.
The attacks have led to a reassessment of how the FBI and the CIA operate and whether they are fulfilling their designated roles.
A Homeland Security department has also been set up to control a number of government security and border agencies as well as co-ordinating operations between the FBI and CIA.
In addition, the FBI is organising a Washington-based "super squad" of some 1,600 agents.
The ease with which people can now move across borders has involved a radical rethinking in intelligence sharing.
The CIA's Director, George Tenet, has assembled a group of more than a dozen countries to form an intelligence coalition.
Hundreds of al-Qaeda and Taleban suspects are detained at Guantanamo Bay
They are increasingly sharing sensitive intelligence on suspected terrorists and initiating arrests.
President Bush has said that his war on terrorism will not end until "every terrorist group of global reach has been found, stopped and defeated".
Intelligence agencies outside the US appear to have had some success in disrupting al-Qaeda since 11 September.
A plot to attack US and British warships near Gibraltar was uncovered in Morocco in May 2002. Three Saudi men received 10-year jail terms for their involvement in February 2003.
In Saudi Arabia 90 people are due to stand trial charged with being members of al-Qaeda. There has also been a string of arrests across Europe.
President Bush has used the state of war to justify the introduction of new legislation that widens his power and strengthens law enforcement.
These moves have triggered a backlash from civil libertarians, who are also angry at new powers permitting the expanded use of secret surveillance
In particular, he has called for the creation of military tribunals to try suspected terrorists who are not US citizens, and has expanded the powers of the law enforcement agencies to fight terrorism.
US officials have detained an undisclosed number of people without trial on immigration violations or on grounds that they may be material witnesses in terrorism trials.
President Bush has also tightened the release of information on the investigation.
A federal order prevents officials from discussing detainees, and in most cases court files have been sealed and hearings are held behind closed doors.
These moves have triggered a backlash from civil libertarians, who are also angry at new powers permitting the expanded use of secret surveillance and wiretaps previously restricted to use against suspected foreign intelligence agents operating in the US.
Attorney General John Ashcroft is leading the intelligence campaign
Attorney General John Ashcroft has rewritten guidelines for the FBI, allowing the agency to monitor political or religious gatherings and the internet outside the course of an active investigation.
Civil libertarians say this erodes safeguards introduced in the wake of FBI excesses in the 1950s and 1960s.
Civil liberties groups have had some success in blocking attempts to detain indefinitely an undisclosed number of people under material witness provisions.
The Bush administration has also run up against some resistance in the courts in its efforts to keep secret the names of those detained on immigration charges.
Assessing the progress
So far two people have been charged, and only one convicted, in connection with 11 September.
Motassadek has been convicted, Moussaoui faces trial over 11 September
Zacarias Moussaoui, detained shortly before 11 September, has been indicted for conspiring with Osama Bin Laden and others to attack the US.
His trial is expected to take place some time in 2003.
Mounir al-Motassadek, arrested in Germany in August 2002, was charged with assisting a group of 11 September hijackers living in Hamburg. He was sentenced to 12 years in prison in February 2003.
Federal authorities say they have found no evidence indicating that any of the people arrested in the US since the attacks played a role in the hijacking plot.
Sheikh Mohammed is the highest profile al-Qaeda suspect captured so far
However the international effort to track down accomplices has yielded far more results.
Al-Qaeda members Mohammed Atef and Ali Qaed Senyan al-Harthi have both been killed by US forces.
Other important suspects - Khalid Sheikh Mohammed, Abu Zubaydah, Mohammed Zammar and Ramzi Binalshibh - have all been arrested.
Closing the net
Intelligence agents have reportedly foiled a number of planned attacks since 11 September, including a highly-publicised plan to blow up the US Embassy in Paris.
The sheer number of arrests around the world - combined with other measures such as the freezing of bank accounts of suspected individuals and organisations - make it more difficult for al-Qaeda agents to operate.
But there are concerns about the methods some countries are using to obtain confessions, and the introduction of anti-terrorist legislation to detain suspects.
It may take many years before the full results of the investigation are released, and much of the detail that has been emerging is based on leaked reports and unattributed sources.