WASHINGTON -- The White House on Thursday defended its decision not to alert Americans before the Sept. 11 attacks that Osama bin Laden's terrorist network might try to hijack airplanes, saying such a warning would have risked shutting down the country's airline system.

The defense came after Democrats led angry calls for President Bush to hand over a top-secret CIA briefing he received in August about the threats. Lawmakers also called for the release of an FBI memo before Sept. 11 that warned headquarters that many Middle Eastern men were training at at least one U.S. flight school.

"It is always a question of how good the information is and whether putting the information out is a responsible thing to do," said National Security Adviser Condoleezza Rice. "You would have risked shutting down the American civil aviation system with such generalized information. You would have to think five, six, seven times about that, very, very hard."

Rice described a series of threats uncovered by intelligence officials, beginning in September of 2000 and reaching a height in the summer of 2001, that dealt mostly with American interests overseas.

Those threats prompted a series of alerts issued by the FBI to law-enforcement agencies and from the Federal Aviation Administration to the nation's airlines and airports, she said.

"The FAA asked security personnel, ground personnel to have a heightened state of alert," Rice said.

She described a flurry of government activities to ward off possible attacks, including strong warnings to Americans to be careful overseas. Bush was told in an Aug. 6 briefing at his Texas ranch about the possible threats, including that the al-Qaida were considering hijackings.

But Rice said the Bush administration never considered alerting the public to a possible hijacking threat at home.

"In the pre-9-11 world, we never even considered issuing a warning," Rice said.

On Capitol Hill, House Minority Leader Dick Gephardt, D-Mo., said: "What we have to do now is find out what the president _ what the White House _ knew about the events leading up to the events of 9-11, when they knew it and, most importantly, what was done about it."

Republican Sen. Richard Shelby of Alabama, vice chairman of the Intelligence Committee, said, "There was a lot of information. I believe and others believe, if it had been acted on properly we may have had a different situation on Sept. 11."

At some point before that day, U.S. intelligence obtained information suggesting that al-Qaida terrorists might hijack an airplane in an attack targeting U.S. citizens, intelligence officials said. The White House and the CIA said there was no suggestion a plane would be flown into a building.

As politically charged congressional hearings loomed, the White House scrambled to shield Bush from damage, and Democrats sought to exploit the first crack in the president's record-setting popularity since Sept. 11.

Bush had no public comment on the developments, but suggested in a closed-door meeting with GOP senators that politics might be at play.

"He said if there had been a strong warning to trust him that he would have reacted quite forcefully," said Sen. Lincoln Chafee, R-R.I., who attended the Capitol luncheon.

"He reminded us this is the political season," Chafee said.

Nine months after the fact, White House aides acknowledged that the CIA told Bush, at his ranch in early August, of intelligence information suggesting a possible hijacking.

The White House said the information was not specific and hardly a surprise _ bin Laden was known to be targeting the United States _ and the intelligence made no mention of suicide hijackings aimed at U.S. landmarks.

Bush was on vacation in Texas when CIA officials passed along the threat, which had already been forwarded to federal agencies.

"That information was then passed on to the airlines, as we customarily do" sometime in June or July, Transportation Secretary Norman Mineta said. "There was really no specificity to the information we received at that time. There was no way we could have, let's say, connected the dots to point to what happened on the 11th of September."

Then in August, the Federal Aviation Administration warned airlines and airports that supporters of bin Laden or other terrorist groups could hijack airplanes, said a government official who spoke on condition of anonymity.

The warning did not include any detailed information and was similar to several FAA notices earlier in the year, said the official.

Some family members of the Sept. 11 victims reacted with anger.

"I believe our whole government let people down," said Bill Doyle of New York City, whose son, Joseph was killed inside the World Trade Center.

Other family members said the government probably did all it could.

"It's time to put aside the anger, the frustration," said Peggy Neff of Hyattsville, Md., who lost her partner of 17 years, Sheila Hein, in the Pentagon attack.

Though Bush and his advisers spent weeks building a public case against bin Laden after the attacks, they never mentioned that U.S. intelligence had picked up threats of hijackings over the summer.

"It's easy to criticize for what did not occur publicly," Homeland Security Director Tom Ridge said in a telephone interview. "But the environment back then was a little different as well, a lot different."

A senior CIA official said it's still not clear whether the information given to Bush was a bona fide hint of the Sept. 11 plot _ or something entirely unrelated to it.

Democrats weren't buying the explanation.

"Why did it take eight months for us to receive this information?" asked Senate Majority Leader Tom Daschle, D-S.D.

He said Bush should turn over the CIA briefing, as well as an FBI memo drafted by an Arizona agent who warned before the attacks of suspicious activity by Arabs at U.S. flight schools. White House officials would not say whether Bush will comply.

Congressional intelligence committees were already trying to determine whether there were any intelligence lapses. Daschle said this week's development will throw the investigation to other panels. Sen. Robert Torricelli, D-N.J., called for a special national commission.

Some Republicans demanded answers, too.

"There were two separate FBI reports plus a CIA warning, none of which were coordinated. The question is, if all three had been connected, would that have led to more vigorous activity? That's the reason why we need the commission to look at it." said Sen. John McCain, R-Ariz.

Gephardt and several members of intelligence panels, primarily Democrats, said they had not been advised of the threats _ although the White House insisted that committee members were told.

The airline industry also said it was caught off guard.

"I'm not aware of any warnings or notifications," said Michael Wascom, spokesman for the Air Transport Association.

White House officials said Bush was steadfast in private that CIA Director George Tenet and FBI Director Robert Mueller had done a good job overhauling their agencies to close the gaps exposed by the attacks. Their jobs are not in jeopardy, officials said.

One Bush associate quoted the president as saying "no one knew" that bin Laden was plotting to make the leap from traditional hijackings to the suicide attacks on the World Trade Center and Pentagon.