Investigation: More detained

SUMMARY:

The United States launched the largest investigation in its history in the wake of the September 11 attacks. While following leads around the country, they have also cast an international net for accomplices.

Investigators identified 19 names used by the suspected hijackers, all of whom have Middle Eastern backgrounds. Dozens of people are in U.S. government custody in conjunction with the investigation and almost 200 are being sought for questioning.

The Bush administration branded Saudi exile Osama bin Laden as the "prime suspect."

UPDATE:

Four people have been arrested in connection with the investigation as material witnesses -- people who may have information about the case and are considered at risk to flee authorities.

•  Summary   •  Update   •  Key questions   •  Who's who   •  Impact

About 80 people also are in government custody, being questioned and in some cases investigated for possible immigration violations. Almost 200 others are being sought for questioning.

Among those being questioned is a Middle Eastern man who was arrested August 17 in Minnesota on an alleged passport violation after he aroused suspicions at a flight school in Oklahoma. He was in custody at the time of the hijackings.

In another development, Philippine investigators said that they warned the FBI six years ago of a terrorist plot to hijack commercial airliners and slam them into the Pentagon. Philippine authorities said they learned of the plot from a close associate of one of the men convicted for the 1993 bombing of the World Trade Center.

KEY QUESTIONS:

Who masterminded the attacks?

How did the hijackers get past airport security and seize control of the planes?

How was U.S. intelligence caught so off guard?

How would proposed changes in surveillance laws affect Americans?

What can be done to prevent similar attacks in the future?

WHO'S WHO:

George W. Bush: U.S. president

Colin Powell: U.S. secretary of state Click here for more.

Condoleezza Rice: National security adviser Click here for more.

John Ashcroft: U.S. attorney general

Robert Mueller: FBI director

George Tenet: CIA director. Click here for more.

Osama bin Laden: U.S. authorities have named bin Laden, a wealthy Saudi exile living in Afghanistan, as the prime suspect in masterminding the September 11 attacks. Click here for more.

IMPACT:

Information gained from the investigation could lead to fundamental changes in U.S. security and intelligence systems, as well as surveillance laws.