Washington ? At least eight Americans were among 21 people killed by suicide bombers in near-simultaneous attacks against three Western compounds in Saudi Arabia — a well-coordinated assault that U.S. intelligence officials said Tuesday they believed was the work of al-Qaida.

The U.S. State Department warned of the potential for “further terrorist attacks in Saudi Arabia” and ordered all nonemergency personnel and family members to leave the kingdom.

The bombings at the walled-off Riyadh housing and office complexes appear to have yielded more American dead and wounded than any single terrorist act since the suicide hijackings of Sept. 11, 2001.

“I believe it’s al-Qaida,” a senior U.S. counterterrorism official in Washington, speaking on the condition of anonymity, said late Tuesday. “This goes to show you that it (the war against al-Qaida) isn’t over yet.”

More than 190 people were reported hurt, including 40 Americans. The Saudis said that in addition to the dead victims, they recovered “nine charred bodies” that officials believed were “those of the terrorists.”

Saudi authorities said gunmen fired on security guards, allowing suicide attackers to drive cars loaded with explosives past checkpoints at the three walled complexes.

‘Despicable acts’

The attacks occurred hours before U.S. Secretary of State Colin Powell was to arrive in Riyadh for talks with Saudi officials. Powell made his visit as scheduled Tuesday and kept all appointments except a meeting with civic leaders, which was scrapped so he could be briefed on the attacks. Powell visited one of the blast sites, a complex for employees of Vinnell Corp.

During a speech in Indianapolis, President Bush labeled the attacks “despicable acts,” saying they “were committed by killers whose only faith is hate.”

Later, in Pierce City, Mo., he told reporters, “There’s a lot of suspicion it is al-Qaida.” But he also added, “We’ll find out — we’re going to find out.”

Counterterrorism and intelligence officials in Washington, however, had few doubts Tuesday that al-Qaida carried out or directed the plot, saying specific intelligence generated before the assaults pointed to the far-flung terrorist network of Saudi exile Osama bin Laden.

They also said the assaults carried numerous al-Qaida hallmarks.

A travel advisory issued by the State Department just 11 days before the bombings said that “terrorist groups may be in the final phases of planning attacks against U.S. interests in Saudi Arabia.” Although the warning did not name al-Qaida, U.S. officials said Tuesday that the advisory was based specifically on intelligence about bin Laden’s network.

Officials cited a number of characteristics that recalled al-Qaida plots. As with the Sept. 11 attacks and the 1998 bombings of two U.S. embassies in East Africa, Monday’s bombings were well-planned, simultaneous assaults against multiple targets — targets likely to yield a high number of U.S. casualties. Only al-Qaida has conducted such operations, they said.

Well-executed plan

U.S. officials also said the preparations must have been exacting, especially given the extraordinary measures maintained by the Saudi security services, which have been on high alert since at least the beginning of the month after detecting possible impending attacks.

“This is obviously not something that they (al-Qaida) put together the night before last,” a U.S. official familiar with the intelligence community’s early assessments of the attacks said.

The U.S. military presence in Saudi Arabia has for years been at the center of bin Laden’s anti-American propaganda.

“I swear to God that America will never dream of security or see it before the infidel’s armies leave the land of Mohammed,” bin Laden said in an October 2001 statement referring to the nation where Islam was born.

The attacks came about two weeks after the U.S. announced it was withdrawing nearly all its 7,000 military personnel from Saudi Arabia.

Seven of the eight Americans killed worked for and lived in the complex established and named for Vinnell, a Fairfax, Va.-based subsidiary of defense contractor Northrop Grumman. Two other Vinnell employees also were killed, the company said, identifying them as citizens of the Philippines.