It took 19 men to hijack four airliners and crash them in New York, Washington and Pennsylvania on Sept. 11, 2001, killing nearly 3,000 people.
But a much larger network of conspirators was involved in planning, funding and carrying out the attacks, and governments around the world have been after them ever since.
Some have been captured or killed and some remain at large or unidentified. A decade later, more than half of those charged in the United States have yet to stand trial.
The following people have been convicted, charged, investigated or named as co-conspirators in legal proceedings connected directly to the Sept. 11 plot and attacks:
Osama bin Laden
WHO: The leader of al Qaeda at the time of the Sept. 11 attacks. Born in Saudi Arabia, he later moved to Afghanistan and then Pakistan.
WHAT: Bin Laden was never indicted for the plot, but was charged by US authorities with the 1998 US embassy bombings in Kenya and Tanzania, as well as in connection with conspiracies to destroy US property and kill Americans. He was also named as an unindicted co-conspirator in a case against Zacarias Moussaoui. All criminal charges against bin Laden were dismissed following his death.
WHERE: Killed by US forces in a private house in Abbottabad, Pakistan, on May 1, 2011, and buried at sea.
WHO: Al-Zawahri, who founded and led the Egyptian Islamic Jihad organization until its 1998 merger with al Qaeda, is a physician from Egypt. He was a top bin Laden deputy at the time of the attacks and now is al Qaeda's leader.
WHAT: Al-Zawahri was named as an unindicted co-conspirator in the Moussaoui case. He was never charged over the 9/11 plot, but has been charged with the 1998 embassy bombings in Kenya and Tanzania.
WHERE: Remains at large. The FBI has offered a $25 million reward for information leading to his capture.
Khalid Sheikh Mohammed
WHO: Believed to have originated the idea for the Sept. 11 plot and to have been operational leader of the attacks. Born in Pakistan and raised in Kuwait, he also claimed a role in other attacks, including the 1993 World Trade Center bombing.
WHAT: Charged by US military prosecutors in 2008 in connection with overseeing training, funding and support for the hijackers and coordinating their US meetings from Saudi Arabia. The case months later was handed to the US Justice Department. Attorney General Eric Holder said in November 2010 the case would be tried in federal court in New York City, but in April he reversed course and sent it back to the military.
The military commission is currently reviewing the charges and will ultimately decide whether the case goes to trial and if so whether the death penalty would be sought.
WHERE: Captured by Pakistani and US officials during a joint operation in Rawalpindi, Pakistan, on March 1, 2003. In US military custody in Guantanamo Bay, Cuba.
WHO: The only Sept. 11 suspect thus far convicted of criminal charges in a US civilian court. A French citizen of Moroccan descent, Moussaoui came to the United States in February 2001.
WHAT: Accused of terrorism conspiracy for allegedly receiving from al Qaeda flight training, funding and instructions to kill Americans alongside the 19 hijackers in the months leading up to the attack.
In 2005, Moussaoui pleaded guilty to all charges in federal court in Virginia. Prosecutors had sought the death penalty, but a jury sentenced him to life in prison without possibility of parole.
WHERE: Detained in August 2001 in Minnesota for violating terms of his visa and charged that December. Serving a life sentence in federal "supermax" prison in Florence, Colorado.
Walid bin Attash, Ramzi bin al-Shibh, Ali Abdul Aziz Ali and Mustafa al-Hawsawi
WHO: Khalid Sheikh Mohammed's co-defendants, accused of playing various support roles in the attacks. Attash and Al-Shibh are both from Yemen. Al Aziz was born in Kuwait. Al-Hawsawi is from Saudi Arabia.
WHAT: Bin Attash is said to have gathered data on airport and airplane security measures. Al-Shibh allegedly tried to enter the US to become one of the hijackers but failing to obtain a visa, instead helped finance and manage the plot from abroad. Aziz Ali is accused of helping fund the plot. Al-Hawsawi is said to have helped the hijackers enter the US and aided the plot once they arrived.
All were charged, along with Mohammed, by military prosecutors and are now awaiting a ruling on how their cases will proceed. All could face the death penalty if convicted.
WHERE: Bin Attash was captured by Pakistani authorities in Karachi on April 27, 2003. Al-Shibh was arrested by Pakistani officials on Sept. 11, 2002, in Karachi. Al-Hawsawi was arrested on March 1, 2003, in Rawalpindi. Ali was captured by Pakistani authorities on April 29, 2003, in Karachi. All are in US military custody in Guantanamo Bay.
WHO: A Saudi citizen, al-Qahtani was identified in the 9/11 Commission Report as one of at least nine "candidate hijackers" who intended to take part in the Sept. 11 attacks.
WHAT: Federal prosecutors said al-Qahtani tried to travel to the United States to become one of the Sept. 11 hijackers, but was denied entry. Military prosecutors initially announced charges against him, Khalid Sheikh Mohammed and four others in 2008 with involvement in the Sept. 11 plot. But in May 2008, a military commission dismissed the charges against al-Qahtani without prejudice, meaning they could be refiled.
Attorneys representing al-Qahtani said the charges were dismissed because the information he provided was obtained using illegal interrogation methods. Al-Qahtani has filed a civil lawsuit along with other Guantanamo Bay detainees seeking relief from what they allege is unlawful detention.
WHERE: Captured by Pakistani forces on the Pakistan-Afghanistan border in December 2001. In US military custody in Guantanamo Bay.
Abdelghani Mzoudi and Mounir el Motassadeq
WHO: The men, of Moroccan descent, were linked to the al Qaeda "Hamburg cell" prosecutors call central to the plot.
WHAT: Mzoudi and Motassadeq were accused of providing material support to the Hamburg cell. Both were tried in German courts on charges of being accessories to the Sept. 11 attacks. Motassadeq was convicted in November 2006 and sentenced to 15 years in prison -- making him the first person in the world convicted on charges stemming from the Sept. 11 attacks.
Mzoudi was acquitted after a judge found there was insufficient evidence to tie him to the plot. His acquittal was upheld on appeal. He was ordered to leave Germany.
WHERE: Motassadeq was arrested in Hamburg on Nov. 29, 2001. He is in a German prison. Mzoudi was arrested in Hamburg on Oct. 10, 2002. He returned to Morocco after his acquittal.
WHO: A Muslim cleric with dual US and Yemeni citizenship. US officials say he is a key leader of al Qaeda in the Arabian Peninsula, based in Yemen.
WHAT: Al-Awlaki preached at mosques in northern Virginia and San Diego attended by three of the Sept. 11 hijackers in the 18 months before the attacks. In 2010, US officials designated him an individual who had committed or was likely to commit a terrorist act and froze his assets.
WHERE: Unknown. Believed to be hiding in Yemen.
(Sources: US department of defense; US department of justice; US department of the treasury; The 9/11 Commission Report; court records.)