The failure to heed these signs is ''an indication of failure to put the pieces together,'' said Gerald B. Kauvar, who was the staff director of the commission headed by Vice President Al Gore on aviation security and safety after the crash of T.W.A. Flight 800 off Long Island in July 1996.

The authorities appeared to draw no lessons from the two attacks in 1994. But one of them, in hindsight, had striking similarities to those of Sept. 11.

That was the December 1994 hijacking of an Air France flight in Algiers. The sponsor of the hijacking was an organization called the Armed Islamic Group, which said it was trying to rid Muslim Algeria of Western influence, specifically from France. Four young Algerians, members of a subgroup called Phalange of the Signers in Blood, commandeered the plane at the airport and ordered it to fly to Marseille, from which they said they wanted to fly to Paris.

But they demanded that it be loaded with 27 tons of fuel -- about three times as much as required for the flight to Paris. The plane was an Airbus A300, which is nearly as large as the Boeing 767's that struck the World Trade Center. The French authorities determined from hostages who had been released and from other sources that the group planned to explode the plane over Paris or crash it into the Eiffel Tower.

After French troops stormed the plane and killed the hijackers, they found 20 sticks of dynamite.

Eight months earlier, in April 1994, a flight engineer at Federal Express who was facing a disciplinary hearing that could have ended his career, boarded a DC-10 as a passenger and stormed into the cockpit with a hammer, hitting each of the three members of the cockpit crew in the head and severely injuring all of them. They wrestled him to the deck and regained control of the plane. Prosecutors said only that the man wanted to crash the plane, but company employees have said he was trying to hit the building in Memphis where the company sorts packages.