A State Department official put the total number of dead at 91, but didn't break that number down by nationality.
A Saudi official said seven Americans were killed. Secretary of State Powell had earlier put that number at ten, but later said it could be lower.
The Interior Ministry official was quoted as saying that attackers used cars packed with explosives in "suicide operations." He said the blasts also killed seven Saudis, two Jordanians, two Filipinos, one Lebanese and one Swiss at the three compounds.
Philippine authorities reported two deaths and Australian officials reported one.
Authorities also found nine charred bodies believed to be those of the suicide attackers, a Saudi Interior Ministry official said.
The bombings, which took place about 11:30 p.m. Monday, constituted the deadliest terror attack on Americans since Sept. 11, 2001, and Powell said the coordinated strike had "the earmarks of al Qaeda."
"Terrorism strikes anywhere, everyone," Powell said. "It is a threat to the entire civilized world."
President George W. Bush denounced the bombings as the work of "killers whose only faith is hate." He vowed to "find the killers and they will learn the meaning of American justice."
"Today's attacks in Saudi Arabia, the ruthless murder of American citizens and other citizens, reminds us that the war on terror continues," Mr. Bush said Tuesday.
The president made the remarks at the Indiana State Fairgrounds, where he was speaking on his tax-cut and economic stimulus plan.
"My thoughts and prayers and those of our fellow citizens are with the families of the victims of yesterday's murders in Saudi Arabia," he said.
There was no claim of responsibility. If the al Qaeda connection is confirmed, it would show that Osama bin Laden's network is still capable of mounting coordinated attacks, even in one of the world's most tightly policed countries.
Before being uprooted in the U.S. war in Afghanistan, the group carried out the Sept. 11 attacks and the 1998 simultaneous car bombings outside American embassies in Kenya and Tanzania that killed 230 people.
Yonah Alexander, the director of the Potomac Institute's Center on International Terrorism and an expert on bin Laden, believes al Qaeda was behind the attack.
"Obviously, I agree with the secretary, because the signature of the attack, directs to Bin Laden and al Qaeda, in terms of their intentions and their capability," Alexander told the CBS Early Show. "This was planned over a long period of time, the simultaneous, very careful attack, and the impact is clearly, very significant.
"The goal of al Qaeda, is to drive out United States' forces and presence from Saudi Arabia, but they have a broader goal to overthrow the Saudi monarchy."
Bin Laden has repeatedly railed against the presence of what he calls "infidel" troops on Muslim holy land.
The Riyadh attack 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 H. Rumsfeld said last week that most would be gone by the end of the summer.
U.S. Central Command spokesman Maj. Brad Lowell said there were no reports of casualties among American service members.
U.S. Ambassador Robert Jordan said at least 40 of the injured were Americans.
The attacks were followed by a smaller bombing Tuesday near the headquarters of a Saudi-U.S. company. No casualties were reported.
A guard at one of the housing compounds in the northeastern section of Riyadh was quoted by the Saudi paper al-Watan as saying that seven cars exploded there, all apparently carrying suicide bombers. At least three bodies could be seen lying on the ground at the compound Tuesday morning.
The force of the blast ripped through multi-story apartments buildings and single-family houses. Facades of five- and four- story buildings were sheared off. Heaps of rubble and blocks of upended concrete surrounded twisted steel bars and knocked downed palm trees. Burned-out hulks that had been cars were still in their parking spots; upended furniture and debris littered a pool deck.
Police vehicles, lights flashing, patrolled the walls of the compounds and kept reporters out. Al-Hamra compound, which suffered one of the worst attacks, was hidden behind 20-foot walls. Surveillance cameras were posted along the walls.
Most of the homes in such compounds are large, single-family villas that would not be out of place in an upper-middle class California. Behind high walls, Westerners can escape Saudi restrictions such as the requirement that women appear outside the home only in enveloping robes and enjoy well-tended parks, swimming pools and sports fields. Their inhabitants tend to be professionals in the oil industry, the financial sector or schools.
Saudi Arabia has a large population of expatriate workers, including about 35,000 Americans.
An intelligence official in Washington said information from the past two weeks indicated al Qaeda had been planning a strike in Saudi Arabia, birthplace of bin Laden and home to Islam's holiest sites.
The FBI is sending a team to Riyadh to help in the investigation of the bomb attacks.
John Pistole, deputy assistant FBI director for the counterterrorism division, will lead what is known as an "assessment team" of a dozen or less agents and technicians, bureau spokesman Bill Carter said Tuesday.
The wealthy gated communities attacked Monday were all in the same part of the city where a May 6 weapons seizure was made and house corporate executives and other professionals from many countries.