MICHELE NORRIS, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Michele Norris.

On Capitol Hill today, allegations of conspiracy and cover-up were flying. The forum was a hearing before the Senate Judiciary Committee, and the subject was a classified military intelligence unit called Able Danger. At the last minute, the Pentagon barred a number of key witnesses from appearing. That decision provoked an angry reaction from both Republican and Democratic senators who wondered aloud what the Pentagon may be trying to hide. NPR's Mary Louise Kelly was at today's hearing and has our report.

MARY LOUISE KELLY reporting:

Today's hearing was originally called to get to the bottom of the Able Danger controversy. The now-disbanded military unit has been making headlines for six weeks now since allegations first appeared that the team turned up valuable information on Mohamed Atta and other 9/11 hijackers more than a year before the terrorist attacks. But from the start of today's hearing it was clear the focus would be on more recent Pentagon activities, namely the move to forbid testimony from officials with direct knowledge of Able Danger. Arlen Specter, the Pennsylvania Republican who chairs the committee, said he can't understand why the Defense Department is, quote, "stonewalling."

Senator ARLEN SPECTER (Republican, Pennsylvania): I was surprised to find that the Department of Defense has ordered five key witnesses not to testify; some of them military, some civilian. That looks to me as if it may be obstruction of the committee's activities, something we will have to determine.

KELLY: Others among the five senators present were quick to chime in. Delaware Democrat Joe Biden called the Pentagon move a big mistake, and Senator Charles Grassley, an Iowa Republican, blasted the Pentagon for trying to prevent Congress from carrying out its oversight responsibilities.

Senator CHARLES GRASSLEY (Republican, Iowa): What's at stake here is whether or not Congress is going to fulfill its constitutional responsibility and whether or not we're going to let people that come up here with a lot of ribbons and a lot of stars on their shoulders just embarrass us and get away with it.

KELLY: A Pentagon spokesman, Major Paul Swiergosz, declined to speak on tape about why, if it has nothing to hide, the Defense Department is blocking testimony. Swiergosz would say only, quote, "We have expressed our security concerns and believe it is simply not possible to discuss Able Danger in any great detail in any public forum."

That left the one Pentagon official who did testify today in the hot seat. William Dugan is a retired Air Force colonel now responsible for the oversight of intelligence activities at the Defense Department. Dugan described his knowledge of the Able Danger program as `very limited,' and his response to many of the questions aimed at him was `I don't know.'

That drew the wrath of Senator Specter. Specter pressed repeatedly on one of the central questions raised by the Able Danger controversy: If Defense officials knew about Atta before 9/11, why wasn't that intelligence shared with the FBI?

Sen. SPECTER: Should it have been disclosed? That's my question. Your last answer was circuitous and not to the point. Should it have been disclosed if it might have prevented 9/11?

Colonel WILLIAM DUGAN (Retired; US Air Force): If it was properly collected, yes.

Sen. SPECTER: Well, was it properly collected?

Col. DUGAN: I don't know, sir.

KELLY: Mark Zaid, a lawyer for two of the witnesses prevented from testifying, used part of his time to try to refocus senators on the big picture. Zaid cited an Able Danger chart--now lost--that allegedly showed Atta and other al-Qaeda suspects.

Mr. MARK ZAID (Lawyer): The focus should be on identifying the current location of the other several dozen possible terrorists that were on that Mohamed Atta chart as to whether or not they are planning to commit terrorist acts against the United States today.

KELLY: But the senators weren't biting. In fact, the focus on the Pentagon and its blocking of witnesses meant that few new details emerged today about the actual substance of the Able Danger operation. A clearly frustrated Senator Specter said he may hold more hearings, but after two hours and 20 minutes, he announced he was suspending today's proceedings, quote, "in the hopes that we'll get some better answers." Mary Louise Kelly, NPR News, Washington. Transcript provided by NPR, Copyright NPR.