WASHINGTON - U.S. officials consistently underestimated credible warnings about al Qaeda terrorist threats on U.S. soil -- some involving aircraft and plots against the World Trade Center and Washington as early as 1998, a congressional inquiry into the Sept. 11 attacks revealed Wednesday.

'Even those of us who couldn't seem to utter the words `intelligence failure' are now convinced of it,'' Sen. Richard Shelby, R-Ala., and the ranking Republican on the Senate Intelligence Committee, said during the first day of public hearings.

''A number of these threat warnings were eerily close to what actually happened,'' said Sen. Bob Graham, D-Fla., the co-chairman of the hearings. ``How is it that so many blocks of information were never analyzed collectively?''

''If a single set of eyes could have seen the emerging plot, leading to more questions, with some good fortune we could have been able to take down this plot before it was implemented,'' Graham said.

NO `SMOKING GUN'

Investigators found no ''smoking gun,'' or detailed, specific information that could have prevented the attacks on the World Trade Center and the Pentagon.

However, Eleanor Hill, staff director of the joint Senate-House investigation, presented a 30-page summary of preliminary findings that contained new, declassified information about threats and warnings. Some examples:

• Intelligence agencies received at least a dozen reports of plans to use aircraft as weapons since 1994.

• Two months before the Sept. 11 attacks, President Bush was warned that bin Laden was planning an attack that could involve hijacking. The briefing from the CIA said: ``We believe that bin Laden will launch a significant terrorist attack against U.S. or Israeli interests in the coming weeks. The attack will be spectacular and designed to inflict mass casualties.''

''Despite these reports,'' Hill said, ``the intelligence community did not produce any specific assessments of the likelihood that terrorists would use airplanes as weapons.''

By December 2000, the FBI and FAA in a classified assessment said their investigations ''do not suggest plans to target domestic civil aviation.'' The FBI considered attacks on U.S. soil ''anomalies'' from terrorists' focus on overseas targets.

National security advisor Condoleezza Rice said in May that before Sept. 11, top officials did not seriously consider the possibility that terrorists would use planes as fuel-laden bombs.

Hill's conclusion: ``The intelligence community made mistakes prior to Sept. 11 and the problems that led to those mistakes need to be addressed and fixed.''

Hill, a former inspector general in the Defense Department, emphasized that many warnings were sketchy and difficult to verify, and that analysts were often ``overwhelmed by a flood of information,''

But, she concluded, ``the totality of information clearly reiterated a consistent and critically important theme: bin Laden's intent to launch terrorist attacks inside the United States.''

Her staff report also criticized intelligence officials for not widely circulating the most serious warnings or devoting more resources to the anti-terrorist effort.

After the embassy bombings in 1998, for example, CIA Director George Tenet wrote a memo to his top deputies about bin Laden and al Qaeda: ``We are at war. I want no resources or people spared in this effort, either inside the CIA or the community.''

But that declaration of war was not widely communicated throughout the government, especially to FBI field agents who could be looking for al Qaeda cells, Hill said.

And from 1998 to the 2001 attacks, the CIA's Counterterrorism Center had only three analysts assigned full time to bin Laden's network. The FBI had one strategic analyst tracking al Qaeda before Sept. 11.

`WHAT WAR?'

''The CIA director had declared war on al Qaeda, but the trumpet call was not widely heard,'' Graham said. 'That raises the question, `What war?' ''

What top officials in the Clinton or Bush administrations knew about some of these warnings was not included in Hill's report -- because the Bush White House and CIA refused to turn over that information, though its substance has been declassified.

''There's an important principle at stake,'' said Anne Womack, White House spokesperson. ``In the past, information that has been provided directly to the president by his closest advisors has not been disclosed in order to preserve the ability of the president to receive the most candid and frank advice.''

Hill disagreed: ``We believe the American public has a compelling interest in this information and that public disclosure would not harm national security.''

Graham and Rep. Porter Goss, the Sanibel Republican who is co-chairman, said they are still negotiating with the White House to release some of that information.

Stephen Push, whose wife Lisa Raines died in the airliner that crashed into the Pentagon, called for a major overhaul of intelligence agencies.

''The time for incremental reform is over,'' Push said. ``If the intelligence community had been doing its job, my wife would be alive today.''

Ron Hutcheson of Knight Ridder contributed to this report.