The compounds housed mostly foreigners working in the country.

The blasts, which also injured over 200 people, are believed to be the work of the same al-Qaida terrorist network responsible for the September 11, 2001 attacks on the World Trade Center in New York and the Pentagon in Washington, D.C.

An orchestrated attack

The bombings coincided with a visit by Secretary of State Colin Powell to Riyadh, the capital of Saudi Arabia, and with recent plans by U.S. officials to withdraw military troops stationed in the region.

Some 5,000 to 10,000 troops stayed on in Saudi Arabia after the 1990-1991 Gulf War. Two weeks before the attacks, however, U.S. Defense Secretary Donald Rumsfeld announced these troops would be transferred to the small Middle Eastern nation of Qatar.

While concerted widespread violence against Westerners has never happened, militant Saudis have attacked Western and government targets, which they accuse of corrupting Islam's holiest territory. The two most important Muslim sites are in Saudi Arabia: Mecca, the birthplace of Mohammed, and Medina, which houses the prophet's tomb.

President George W. Bush has pledged to find those responsible for the attacks. "The United States will find the killers, and they will learn the meaning of American justice," he said.

Americans in Saudi Arabia

An estimated 60,000 Westerners live in Saudi Arabia, a small portion of the 6.4 million resident foreigners and approximately 17 million Saudis.

Western workers in the kingdom often live in guarded compounds, separated from the general Saudi population. The complexes are designed to provide security and more freedom from cultural and behavioral rules stipulated by the country's official religion, a strict brand of Islam known as Wahabbism.

Two of the targets, the Al Hambra and Jadawel housing complexes, were luxury-style compounds complete with pools, clubhouses, and recreation facilities.

Another target was a residential facility run by the Vinell Corporation, part of U.S. defense contractor Northrup Grumman, that provides training for the Saudi National Guard. A fourth target was the Saudi Maintenance Company also known as Siyanco.

The population of the targeted residential areas was about 40 percent Saudis and other Arabs and 60 percent other nationalities, including Americans and Britons, the Washington Post reported.

After the attacks, the U.S. government ordered all "nonessential" officials to leave the country.

Warning signals?

The bombings came one week after Saudi police raided a Riyadh apartment filled with explosives. The raid failed to capture al-Qaida operatives thought to be behind the stash.

"We had indicators that they were planning something," a senior U.S. government official told the New York Times. "We didn't know exactly what."

U.S. officials in Riyadh said they had approached the Saudi government for increased protection at residential areas in the city, but the request had not yet been granted.

Saudi Foreign Minister Prince Saud al-Faisal said his government had come close to preventing the attacks but could not stop them.

Search for the killers

Investigators on Tuesday went through the rubble at the scene of the bombing, searching for clues to the attackers' identities.

Since September 11, 15 Americans have been killed in large-scale terrorist attacks -- eight in Monday's bombing and seven in the October 2001 nightclub bombing in Bali, Indonesia.