At least ten Americans, and one Australian were killed; an estimated 50 people were injured, including at least 30 Americans.
Monday night's terror attacks in Riyadh came in quick succession, and were followed by a fourth explosion early Tuesday outside the headquarters of a joint U.S.-Saudi owned company in Riyadh.
Australian Foreign Minister Alexander Downer told the Australian Broadcasting Corp. that three people were confirmed dead but gave no other details.
The American death toll was announced by Colin Powell as he arrived in Riyadh Tuesday, a trip he decided not to cancel despite the bombs which exploded only a few hours earlier.
"Terrorism strikes everywhere and everyone," Powell told reporters gathered in Riyadh. "It is a threat to the civilized world."
No one has claimed responsibility for the attacks, but Powell says the bombings have "the earmarks of al Qaeda."
"I believe al-Qaeda has been weakened, but it has not been destroyed," said Powell, adding that the U.S. will not be deterred "from pursuing the interests of peace around the world."
Powell was greeted on his arrival by Prince Saud, the Saudi foreign minister, who expressed his sorrow and vowed to cooperate with the United States in fighting teerrorism.
"These things happen everywhere," said Saud.
Powell, who already has visited Israel, the West Bank, Egypt and Jordan on a Mideast tour, is in Riyadh to meet Saudi leaders to seek help in harnessing militant groups and in promoting Palestinian reform.
An intelligence official in Washington said information from the past two weeks indicated al Qaeda was planning a strike in Saudi Arabia.
Saudi officials also have said al Qaeda was planning attacks in the oil-rich kingdom, which is Osama bin Laden's birthplace and home to Islam's holiest sites. Saudi Arabian men also made up 15 of the 19 Sept. 11 hijackers.
CBS News National Security Correspondent David Martin reports Saudi authorities last week raided a safe house in Riyadh, found 800 pounds of explosives, and issued an alert for what they said are 19 suspected militants. The Saudis say the 19 were trained at al Qaeda terror camps and had come to the kingdom for the purpose of launching suicide bomb attacks.
The blasts came as the United States is pulling out most of the 5,000 troops it had based in Saudi Arabia, whose presence fueled anti-American sentiment. Defense Secretary Donald Rumsfeld said last week that most would be gone by the end of the summer.
Bin Laden has used the presence of U.S. soldiers in the kingdom - the birthplace of Islam - as a rallying call for attacks on U.S. interests worldwide.
In Monday night's attacks, gunmen in three cars shot their way into the three residential compounds before setting off explosives in the vehicles, a Saudi official said on condition of anonymity.
The official said it was not known if the gunmen killed themselves in the blasts or fled.
Smoke rose into the night sky from one of the attacked compounds, located in the Garnata neighborhood in eastern Riyadh, and a helicopter circled overhead, scanning the ground with a searchlight. Hundreds of anti-riot police and members of the elite National Guard were evacuating the area and sealing it off as ambulances rushed in.
"We don't know how many are injured, but we received 50 and the number is growing," an official at the National Guard Hospital in Riyadh told The Associated Press by telephone, without identifying himself. "We're very busy, we are receiving a lot of casualties."
The wealthy gated communities that were attacked house corporate executives and other professionals. About half of the residents were Westerners and the rest were Saudis and other Arabs, a Saudi official said.
State Department officials said the American school in Riyadh likely will be closed Tuesday, and advised Americans to remain at home until further notice. Earlier this month, it had advised Americans earlier against traveling to Saudi Arabia because of increased terrorism concerns.
There are about 35,000 U.S. citizens in Saudi Arabia.
Justice Department and FBI officials said they were monitoring the situation but had no immediate indication that other attacks might be planned against U.S. interests at home or abroad.
Witnesses at the al-Blaidh compound in the Garnata neighborhood said the force of the blast shook nearby buildings and rattled windows. Witnesses also reported hearing gunfire moments before the car exploded. The compound is owned by Riyadh's deputy governor, Abdullah al-Blaidh.
Mete Kavuncu, a worker from Turkey, said he heard an explosion about 11 p.m. in northeast Riyadh.
"I felt a shake. It was a strong explosion," Kavuncu said. Shortly afterward, colleagues called to say that one of his company's employees was wounded in the attack on the al-Hamra compound.
An American who lives in one of the targeted areas compounds told the AP in an e-mail exchange from Riyadh that there was extensive damage to property.
Three Boeing Co. employees were slightly injured by flying glass, said Boeing spokesman Bob Jorgensen. They are among a group of 12 Boeing instructors training Saudi Air Force on operating Airborne Warning and Control System (AWACS) jets, the spokesman said in Seattle.
The fourth blast went off at the headquarters of the Saudi Maintenance Company, also known as Siyanco, early Tuesday morning. The company is a joint-owned venture between Frank E. Basil, Inc., of Washington, and local Saudi partners, the officials reported.
A previously unknown Saudi group, the Mujahedeen in the Arabian Peninsula, earlier vowed on an Internet site to strike against American targets worldwide but it was not clear whether the explosions in Riyadh were linked to the group.
Last week, a senior Saudi security official said suspected terrorists were receiving orders directly from bin Laden and had been planning attacks in Saudi Arabia targeting the royal family as well as American and British interests. The prime targets were the defense minister, Prince Sultan, and his brother, the interior minister, Prince Nayef, the official said.
On May 6, Saudi security forces seized a large cache of weapons and explosives in Riyadh when searching for a number of suspected terrorists, an unidentified government official told the state-run Saudi Press Agency.
At least 19 men, including 17 Saudis, an Iraqi holding both Kuwaiti and Canadian citizenship, and a Yemeni, were being sought in connection with the terror plots, the agency reported.
Their names and pictures were shown on state-run Saudi television, and a reward of more than $50,000 was offered to anyone turning in any of the suspects.
A week earlier, an American civilian working for the Saudi Royal Navy was attacked and slightly injured in eastern Saudi Arabia.
In 1996, a truck bombing killed 19 Americans at the Khobar Towers barracks in Dhahran.
In 1995, a car bomb exploded at a U.S.-run military training facility in Riyadh. Seven people died, including five American advisers to the Saudi National Guard. The Islamic Movement for Change and two smaller groups in the region claimed responsibility.