More Attacks Feared as Probe Intensifies

W A S H I N G T O N, Sept. 20 -- Government officials are warning of a possible second wave of terrorist strikes in the United States as the FBI tries to determine whether some of the hijackers involved in last week's attacks were traveling under stolen identities.

MORE INVESTIGATIVE NEWS:

• FBI Uncertain of Hijackers' Identities• Terrorist Manhunt Continues• New Arrest Made• Possible Terrorist Stock Trading Probed

"We do expect more attacks," a senior Bush administration official said today. "We're at the point where anything is possible, but nothing can be predicted."

Law enforcement and intelligence sources tell ABCNEWS the FBI believes there is a strong possibility a network of terrorists is still in place in the United States preparing to launch additional attacks.

"We … have continuing concerns about the people responsible for Tuesday's attacks and information that does suggest they might have planned further attacks," said one senior intelligence official, who asked not to be identified. "But we don't have any intelligence that gives us specific dates or methods for those attacks."

Capitol Hill sources who have been briefed by the CIA tell ABCNEWS that intelligence points to an "umbrella plan" to destabilize the country.