List of Foiled Plots Puzzling to Some

U.S. Attorney General Alberto R. Gonzales declined to discuss the White House-produced list in detail or whether the Justice Department was consulted in its formation. "The fact that they're not on that list doesn't mean those other successes weren't important," Gonzales said.

The Department of Homeland Security did not respond to multiple requests for comment. During most of the domestic plots on the White House list, the threat index remained at yellow. For example, one of the newly disclosed attacks was a disrupted mid-2002 plot to use aircraft hijacked overseas to target the Library Tower in Los Angeles.

During the summer of 2002, the Department of Homeland Security did not raise the threat level. U.S. officials issued warnings about possible terrorist attacks, but none involved hijacking commercial airplanes.

That summer, the FBI issued a warning about threats to use fuel trucks to attack Jewish schools or synagogues, small airplanes to carry out suicide attacks in the United States, terrorists allegedly trying to obtain "offensive scuba diver capability," and a general warning about landmarks such as the Statue of Liberty and the Brooklyn Bridge.

"The problem with these lists is that we don't know the criteria," said Bruce Hoffman, a Rand Corp. terrorism expert. When the incidents do not correspond to elevated threat levels, "it runs the risk of 'Were we just crying wolf then?' This is animpressive compendium of actual attacks, but what about the other ones?"

One well-known series of events not on the list is the terror threat over the New Year's holiday in 2003-2004, when the U.S. government took the unprecedented step of canceling dozens of commercial airline flights from London and Paris over concerns that terrorists were targeting specific flights.

But that may be because the threat was later discounted, according to intelligence officials. A CIA contractor at the time had analyzed bar codes on al-Jazeera network videotapes and concluded that the codes could be flight numbers targeted by terrorists, former intelligence officials said.

One former official, David Stone, who served as chief of the Transportation Security Administration from late 2003 until the spring of 2005, helped coordinate the nation's response to the threat against certain "flights of interest" between Christmas 2003 and New Year's 2004.

The nation's alert level was elevated to orange at the time, and the United States asked Britain and France to cancel U.S.-bound flights from London and Paris at a cost of hundreds of thousands of dollars each. Passengers on flights that did take off were heavily screened before departure and upon arrival. The incident strained relations between the United States and Britain, France and Mexico as U.S. officials pushed those reluctant countries to put armed air marshals aboard flights.

Stone and other top TSA officials were not made aware that the intelligence used to take these actions was later discounted.

The high-profile concern over "flights of interest" was based upon analysis completed by a CIA contractor that said it found specific flight numbers embedded in bar codes on videotapes of al-Jazeera broadcasts, according to former and current intelligence officials who spoke on the condition of anonymity. One of the bar codes was numerically identical to the date of an attack on Istanbul, so the contractor's analysis of other bar codes convinced CIA officials that the other codes signified flight numbers, intelligence officials said.

Staff reporters Dan Eggen, Dana Priest, Spencer S. Hsu and Dafna Linzer and researcher Julie Tate contributed to this report.