WASHINGTON (AP) - Intelligence agencies failed to anticipate terrorists flying planes into buildings despite a dozen clues in the years before the Sept. 11 attacks that Osama bin Laden or others might use aircraft as bombs, a congressional investigator told lawmakers Wednesday as they began public hearings into the attacks.

Just a month before the attacks, intelligence agencies were told of a possible bin Laden plot to hit the U.S. Embassy in Kenya or crash a plane into it.

The preliminary report by Eleanor Hill, staff director of the joint House and Senate intelligence committee investigation of the terrorist strike, showed authorities had many more warnings about possible attacks than were previously disclosed.

The reports were generally vague and uncorroborated. None specifically predicted the Sept. 11 attacks. But collectively the reports "reiterated a consistent and critically important theme: Osama bin Laden's intent to launch terrorist attacks inside the United States," Hill said.

Despite that, authorities didn't alert the public and did little to "harden the homeland" against an assault, she said. Agencies believed any attack was more likely to take place overseas.

Just two months before the attacks, a briefing for senior government officials said that, based on a review of intelligence over five months, "we believe that (bin Laden) will launch a significant terrorist attack against U.S. and/or Israeli interests in the coming weeks."

"The attack will be spectacular and designed to inflict mass casualties against U.S. facilities or interests. Attack preparations have been made. Attack will occur with little or no warning," it said.

Hill read most of her 30-page report to House and Senate members sitting together in what is believed to be the first joint investigation by standing congressional committees. The committees have been meeting behind closed doors since June to examine intelligence failures leading up to the attacks and recommend changes.

Sen. Jay Rockefeller, D-W.Va., said the report revealed "far too many breakdowns in the intelligence gathering and processing methods."

"Given the events and signals of the preceding decade, the intelligence community could have and in my judgment should have anticipated an attack on U.S. soil on the scale of 9/11," he said.

Pressed by Rep. Ray Lahood, R-Ill., about whether agencies had enough information to have prevented the attacks, Hill said it was possible, but there were no guarantees.

Details of intelligence about terrorist use of airplanes could embarrass the White House. After questions were raised in the spring about what President Bush knew about terrorist threats before Sept. 11, national security adviser Condoleezza Rice said the threats were vague and uncorroborated.

"I don't think anybody could have predicted ... that they would try to use an airplane as a missile," Rice said then. "Had this president known a plane would be used as a missile, he would have acted on it."

Hill outlined 12 examples of intelligence information on the possible terrorist use of airplanes as weapons, beginning in 1994 and ending with the Nairobi plot in August 2001.

In August 1998, U.S. intelligence learned that a "group of unidentified Arabs planned to fly an explosive-laden plane from a foreign country into the World Trade Center," says the report. The report was given to the Federal Aviation Administration and FBI, which took little action. The group may now be linked to bin Laden, the report says.

Other intelligence suggested that bin Laden supporters might fly an explosives-laden plane into a U.S. airport, or conduct a plot involving aircraft at New York and Washington, the report said.

While generally aware of the possibility of these kinds of attacks "the intelligence community did not produce any specific assessments of the likelihood that terrorists would use airplanes as weapons," the report said.

The FBI on Wednesday underscored the need for continued vigilance by law enforcement agencies, confirming that it sent a routine reminder to police around the country in the last day or so. The reminder said that al-Qaida might consider the use of aircraft in another act of terrorism against the United States and could rely on non-Arabic individuals to do so.

Hill also said that between May and July 2001, the National Security Agency reported at least 33 communications indicating a possible, imminent terrorist attack. Asked why intelligence agencies didn't do more about the terrorist threats, Hill said they have complained about a lack of resources and the massive amount of intelligence they were receiving. "They were overwhelmed by almost a flood of information," she said.

Senior CIA officials noted Hill's report also recognized their efforts to report on the immediacy of the threat from bin Laden before Sept. 11 and did not look to assign blame on U.S. agencies.

Hill stressed the investigation is continuing and a future report will deal with what was known about the 19 hijackers before the attacks.

She also noted that CIA Director George J. Tenet has declined to declassify information on two issues looked at by the inquiry: References to intelligence agencies supplying information to the White House, and details of an al-Qaida leader involved in the attacks. That leader is believed to be Khalid Shaikh Mohammed, the alleged mastermind.

Hill said the White House and Tenet believe "the president's knowledge of intelligence information relevant to this inquiry remains classified" even when the information itself is declassified.

Also Wednesday, two spouses of Sept. 11 victims urged the committees to fix intelligence shortcomings that allowed the attacks.

"Our loved ones paid the ultimate price for the worst American intelligence failure since Pearl Harbor," said Stephen Push, whose wife died aboard the plane that crashed into the Pentagon.