I just came across the front page of the Sun-Times from October 26, 2000. It carried this banner headline:
US COLE ATTACK
A 40 x 40-foot hole had been blown in the side of the Cole by a suicide bomber during a refueling stop in Yemen two weeks earlier. Seventeen sailors died. The article reported that a Pentagon intelligence analyst had warned of “impending terrorist attacks” but the warning wasn’t passed along to military commanders. A later warning, of “imminent terrorist activity” in Yemen, wasn’t acted on.
The thing is, the attack on the Cole was itself a warning: al Qaeda was at war against the United States. If taken as seriously as the 9/11 attacks 11 months later, it might have prevented them. But it was not.