House Democratic Leader Nancy Pelosi said events reinforced the need to implement the recommendations of an independent 9/11 commission, a reminder of one of her party's main campaign promises.

In Ohio, Republican chairman Bob Bennett accused the Democrats' senatorial challenger of voting against funds "for the very types of programs that helped the British thwart these vicious attacks."

Homeland Security deputy Secretary Michael Jackson said his agency had known for several days of the unfolding plot but waited for a signal from the British to announce it.

The decision to raise the terror level for flights from Britain indicated a severe risk of terror attacks. The "code red" change requires airlines to provide the government with an advance list of passengers aboard affected flights. Previously, passengers names had to be provided within 15 minutes after takeoff.

All other flights to and within the United States were put under an "orange" alert, one step below red, but an escalation from the "yellow" status that had been in effect.

Administration officials sought to reassure the traveling public at the same time they imposed heightened security restrictions.

"Today, air traffic is safe, and air traffic will remain safe precisely because of the measures we are adopting today," Chertoff said.

Gov. Arnold Schwarzenegger of California deployed 300 Guard troops to at least three large airports — in Los Angeles, San Francisco and Oakland — where direct flights from Europe were scheduled to arrive.

In Massachusetts, Gov. Mitt Romney announced he would activate Guard troops for airline security duty for the first time since the terror attacks of 2001, and New York Gov. George Pataki said Guard troops would be used there, as well.

White House spokesman Tony Snow said Bush had been briefed in advance of the events and had approved raising the alert to red on flights from England.

Senior lawmakers also had received advance word. Several said they had been briefed by Homeland Security or CIA officials as early as Monday.

Officials said the plotters had been planning a test run within two days to see whether they could smuggle the equipment they needed aboard the flights. The actual attack would have occurred within days.

Chertoff, interviewed on CNN, did not challenge the account.

"It's not uncommon that these kinds of plots almost always have a dry run or a casing element before the actual plot is carried out," he said.

The attacks of Sept. 11, 2001, "had dry runs as well," he said.

Rep. John Kline, R-Minn., was the only lawmaker to attend a closed-door briefing in the Capitol.

"This was a very close call," he said of the aborted plot.

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Associated Press writer Katie Shrader contributed to this report.