RIYADH, Saudi Arabia -- The deadly car bombings of residential compounds in Riyadh could be followed by more attacks on Americans, U.S. officials warned Thursday.
A six-member FBI team came to the kingdom Thursday to determine what is needed to help in the investigation into Monday's attacks, which killed 34 people, including eight Americans.
Thursday night the State Department issued a new warning to Americans in Jiddah, Saudi Arabia.
"The U.S. Consulate General in Jiddah has received an unconfirmed report that a possible terrorist attack in the Al Hamra district of Jeddah may occur in the near future. While we cannot certify the credibility of the threat, in light of recent events this information is being shared with the American Community. Some Consulate families resident in the Al Hamra district temporarily relocated to different quarters," the warning said.
"U.S. citizens are encouraged to maintain a high level of vigilance and to take appropriate steps to increase their security awareness."
As the investigation began, Robert Jordan, the U.S. ambassador to Saudi Arabia, said he expects "good cooperation" from the Saudis.
"I think both parties have learned from past experiences," he told journalists at his residence. "There is a sincere, good faith effort on both parts to cooperate, to work as partners to share information without jealousy or petty bureaucracy."
The attack "was, if not the Saudi Sept. 11, it was certainly the Saudis' Pearl Harbor," added Jordan.
"This is a battleground rather than simply a nice place for civilians to live," he said. "Everyone has to confront that."
Saudi cooperation with the U.S. investigators could be critical. Some U.S. experts worry the Saudis will limit American access to suspects and evidence, as they did after the 1996 bombing of the Khobar Towers military dormitory that killed 19 U.S. service members.
Before the latest attacks, American intelligence uncovered evidence that groups were targeting Westerners and Americans, but there had been no specifics on targets or dates, said a U.S. official who spoke on condition of anonymity.
The official said the U.S. assessment was based on a "calculation and a surmise" and educated guesses "because most intelligence doesn't draw you a clear picture."
The "picture we had seen before would have indicated that there was going to be a stream of attacks, and so we have confidence that has begun," the official added.
The official said the U.S. government is communicating its concern to Americans in the kingdom and is asking the Saudi government for help.
Ahead of Monday's car bombings, intelligence officials had feared an attack was coming.
Jordan sent three letters to the Saudi Interior Ministry requesting enhanced security at residential compounds.
The first letter was sent on April 29, shortly after the Americans received intelligence suggesting imminent attacks aimed at Western interests, soft targets and housing compounds.
The ambassador renewed his concerns in another letter sent May 7, a day after a raid on a terrorist safe house near Jadawal, one of the three compounds attacked Monday.
The Washington Post reported on its Web site Thursday that the view from the second floor of the safe house allowed the presumed attackers to case the compound and study the habits of Saudi air force guards in a tower at the perimeter. The attackers reportedly struck after an air force guard left his machine gun in the tower and went below to have tea with other unarmed security guards.
The newspaper also said that the attack caused no casualties among residents at Jadawal because its security chief had recently begun keeping closed the outter-most of two gates at the compound, which formerly had stood open from 6 a.m. to midnight, about 30 minutes after the attacks occurred.
The delay caused by the closure of the first gate, allowed guards at an inner gate to secure it, forcing the attackers to detonate their explosives-laden vehicle outside the compound. Two security guards and five attackers died.
An investigation made the Americans conclude there was a good chance that Jadawal would be a target, and they sent a third letter to the ministry on May 10 specifically citing that compound.
Prince Bandar bin Sultan, the Saudi ambassador to Washington, said Jordan had asked "for increased security at a certain compound."
"We have passed it to the right authority, and that compound that he was concerned about was the only place that the evil people who did this attack did not cause injuries except killing the Saudi guards," Bandar told CBS News.
"Our security agencies, ... the air force came to the conclusion there were adequate measures there," he said Thursday on ABC's "Good Morning America."
But Bandar added, "There is no 100 percent security when a determined, crazy, evil person is determined to die."
Jordan said there was a "very clear suggestion that this attack was aimed at undermining the (Saudi) government as much as it was aimed at American interests."
Two of the compounds housed employees of the Saudi national guard, headed by Crown Prince Abdullah, and workers in the air force run by the Defense Ministry that is headed by Prince Sultan. Both are brothers of King Fahd. The third complex is owned by the deputy governor of Riyadh.
Jordan said bin Laden and al-Qaida have lost support in the kingdom because of the attacks.
"My guess is, if I were to say today how popular they are, they are much less popular than they were a week ago," the ambassador said.
"I think in some ways they have hurt themselves here," added Jordan. "They have gone too far and they have soiled the nest. It may actually be their downfall."