Al Qaeda (search) attacks against American and Western targets are "likely" and "attacks in the U.S. cannot be ruled out," the FBI said in a special edition of its bulletin to law enforcement agencies throughout the country.
In the bulletin, issued on Friday, the FBI said last week's homicide car bombings in Riyadh (search), Saudi Arabia, indicate "further refinement in Al Qaeda operational capabilities.''
"The May 12 bombings in Saudi Arabia indicate that the Al Qaeda network remains active and highly capable," the FBI said. "The U.S. Intelligence Community assesses that attacks against U.S. and Western targets overseas are likely; attacks in the United States cannot be ruled out.''
But officials noted Monday that they have no credible information about a specific threat from Usama bin Laden's terror network, nor has anything occurred since the bulletin was issued to indicate that an attack is imminent.
White House spokesman Ari Fleischer said security officials are reviewing intelligence intercepts.
Fleischer said in a televised interview Tuesday that there's been significant "chatter in the system." He said the decision on whether to raise the national terror alert threat level is a "day-to-day call."
The national alert status remained unchanged at "yellow" Tuesday -- an elevated level, but that's in the mid-range of the five-tier warning scale below orange and red.
U.S. intelligence officials are used to increases in the chatter level. Fleischer said "we've seen this before," but that the government wants to remain "vigilant."
President Bush said Monday that the United States is slowly but surely dismantling Al Qaeda, despite a wave of attacks in Saudi Arabia, Morocco and Israel. He pledged to pursue Mideast peace but acknowledged it's "going to be a bumpy road."
"This is still a dangerous world we live in, and clearly the attack in Saudi Arabia means we have to be alert here at home," Bush said.
The FBI, in its bulletin, said "state and local law enforcement agencies" should remain vigilant to potential indicators of pre-operational planning, such as target surveillance and acquisition of explosives material.
"The Al Qaeda network and other international terrorist groups have demonstrated the ability to plan and carry out complex, simultaneous attacks against multiple targets."
The FBI is helping Saudi authorities in the investigation of the bombings on three Saudi housing compounds that killed 34 people, including eight Americans, last week. Al Qaeda is thought to be responsible for that bombing, as well as another series of bomb attacks Friday in Casablanca, Morocco, that killed 41 people.
The bulletin describes the Riyadh bombings in detail and lists similarities between the attacks on the compounds:
• The use of two vehicles, including a sedan-type passenger automobile followed by a larger sport-utility-type vehicle or small truck capable of carrying a VBIED (search) (vehicle-borne improvised explosive device), as well as additional armed operatives.
• The use of "dismounted'' gunmen to engage guards and penetrate security countermeasures.
• pre-operational planning and surveillance
• targeting of "soft'' or lightly-secured sites
• a focus on causing mass casualties
"Further, these attacks suggest that Al Qaeda may be deterred by enhanced security and changes in the security countermeasures adopted by potential targets," the FBI said in the bulletin. "Al Qaeda operatives will adapt their targeting to maximize the likelihood of operational success, selecting softer targets if more hardened sites are considered too difficult to attack successfully.''
The FBI has previously warned that terrorists could strike apartment buildings, hotels, restaurants and businesses.
Other similar attacks blamed on the terror network include the October 2002 bombing of a nightclub district in Bali, Indonesia, which killed almost 200 people, and the homicide bombings of an Israeli-owned beach hotel in Kenya, which killed 12. In the Kenya attack, two missiles narrowly missed an airliner carrying Israeli vacationers.
Fox News' Anna Stolley and The Associated Press contributed to this report.