"2.an indication or warning of probable trouble"
Considering people had just watched 2 towers crash down from attacks, a successful attack against the Pentagon, and a near-miss against what we think was the Whitehouse, and having no indication another one *wasn't* going to happen, the there was overwhelming reason to believe that an "indication or warning of probable trouble" was correct.
As time went on, people have moved from a "probably" state to a "possible" state, and perhaps even to a "improbably" state now. However, such was not the case right after the attacks.
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