Investigators sifted through mounds of concrete and steel rubble Wednesday at a housing compound in northeastern Riyadh, Saudi Arabia, where one of several car bombs that exploded Monday night left a crater 20 feet wide and 3 feet deep.

Saudi authorities raised the death toll in the attacks to 25, including eight Americans.  Three housing compounds where foreign residents lived were devastated. Nine homicide bombers also were killed.

Saudi and Western authorities linked Al Qaeda to the bombings and warned that more terrorism could lie ahead.

"We can't be sure this is the only attack being planned out there," Robert Jordan (search), U.S. ambassador to Saudi Arabia, told Fox News Wednesday.

The bombers apparently were part of a known Al Qaeda cell headed by a veteran Saudi militant who trained in Afghanistan, The Washington Post reported Wednesday.

Saudi officials said the cell was formed in Saudi Arabia after the Sept. 11 attacks and was led by Khaled Jehani, a 29-year-old who took over after the capture last November of Abd al-Rashim al-Nashiri, a key planner in the USS Cole attack (search) in Yemen in October 2000.

The United States on Tuesday ordered most nonessential diplomats and family members to leave Saudi Arabia and dispatched FBI investigators to Riyadh to help with the investigation.

About 2,000 Saudi civil-defense workers searched for evidence of the attackers' identities and methods Wednesday.

President Bush spoke to Saudi Crown Prince Abdullah on Tuesday night, pledging U.S. support for the kingdom's fight against terrorism, Saudi media reported.

The U.S. Embassy in Riyadh was closed Wednesday. U.S. diplomats said the embassy was still "trying to figure out how many Americans have died."

The eight deaths were the highest American death toll in terror attacks since Sept. 11.

Seven of the Americans killed were employees of a local subsidiary of the Fairfax-based Vinnell Corp. (search), contracted to train the Saudi Arabian National Guard, the Post reported.

Northrop Grumman, which owns Vinnell, said two Filipino employees also were killed and others were hospitalized, two in serious condition.

Another 200 people were wounded, most not seriously, and 40 of these were believed to be Americans, according to Saudi officials.

"These despicable acts were committed by killers whose only faith is hate, and the United States will find the killers, and they will learn the meaning of American justice," Bush said.

Jordan said about 16 Americans were still in the hospital, several of whom "are in very bad condition. ... It's a horrendous sight."

"It would remind you of Oklahoma City," Jordan said after visiting the bombing sites. "It was an absolutely horrific event."

A German woman living in the al-Hamra compound said she and other residents "feel really scared."

"Things are going to get worse for us now, " she added.

Asked whether she planned to leave Saudi Arabia, where she had lived for eight years, she replied: "We haven't decided that yet, but it's a possibility."

The Saudi government said the attacks were connected to 19 Al Qaeda operatives who engaged in a gunfight with police in Riyadh on May 6 as authorities seized a huge weapons cache.

"Some of [the attackers] were members of the group that was sought a few days ago, the 19 fellows whose pictures came out in the press," said Prince Turki al-Faisal, Saudi Arabia's ambassador to Britain and a former Saudi intelligence chief.

The 19 escaped, although one later turned himself in. Among them were 17 Saudis, a Yemeni and an Iraqi with Kuwaiti and Canadian citizenship.

Interior Minister Prince Nayef said they likely took orders directly from Usama bin Laden, the alleged mastermind of the Sept. 11 attacks.

The prince also did not rule out the possibility of more attacks.

"This is life, and incidents occur in every country and we are in a period of anxiety and terror acts. The kingdom is one of the countries being targeted," he told the Saudi newspaper Okaz.

Sen. Pat Roberts, R-Kan., chairman of the Senate Intelligence Committee, said there was no question that the intelligence that led to that warning was related to the attacks.

"There was very significant reporting across the community that plans not only were being made, but the decision to attack had been made," he said. "It was just a matter of who and when and where."

"This is not unexpected — it's disappointing, but it's not unexpected," Rep. Peter Hoekstra, R-Mich., a member of the House Select Intelligence Committee, told Fox News Wednesday.

"There's no doubt there are things that we knew — pieces of the puzzle where now in hindsight, we're saying 'whoa — we could've connected those things.'"

A State Department spokesman also said Tuesday that the attack was no surprise.

Saudi Foreign Minister Prince Saud al-Faisal said on NBC's Today show Wednesday that "there was news coming from everywhere that they were planning a major attack and we had established a committee with the United States to see what we could do both of us in order to prevent this attack from happening."

"We came close, we came indeed very close to doing that, but unfortunately they were able to do their damage," he said.

Jordan said the Saudis had shared information with the United States about the increased Al Qaeda activity.

Most of the residential compounds did not receive increased security, although they had requested it.  The Saudi guards who were installed at one compound were all killed in Monday's blasts.

"I certainly expect we'll demand they'll allow us to investigate as thoroughly as we need," Jordan said.

Earlier this month, the State Department advised Americans to avoid travel to Saudi Arabia because of increased terrorism concerns.

"At this time, I don't think it would be prudent for Americans to plan a visit here," Jordan said.

The targeted compounds — al-Hamra, Jadawal and Vinnell — were within 10 miles of each other in northeastern Riyadh and are home to Western business executives, oil industry professionals and teachers.

Saudi Arabia has a large population of expatriate workers, including about 35,000 Americans.

Around 11:30 p.m. Monday, witnesses reported gunfire and a series of explosions.

It took the bombers less than a minute to kill the guards, open an iron gate, drive up to the buildings and detonate the explosives.

"They had to know where the switches were," said a U.S. official traveling with Secretary of State Colin Powell, who visited the area.

The Post reported that Saudi officials said at least some of the attackers wore Saudi Arabian National Guard uniforms and approached the compounds in the same type of vehicles often used by residents and guards.

"I hope those people who were responsible for these acts face the full weight of the law, and if they are men of religion, that when they depart this world that they are punished in the next world, too," said al-Hamra resident Graham Bull, a teacher at the British School who suffered minor injuries.

The Associated Press contributed to this report.