"no known attack is planned" means exactly what it says - the US government has no knowledge of an attack in the planning stages. Of course there could be one that is being planned, but if there is, the government doesn't know about it.
Why should you not fear? Because then you'd live your entire life in fear for something that may or may not occur. If you really are scared, then help whatever government you are a citizen of fight terrorists either by getting a job as an analyst, a police officer or even by being an ordinary citizen who makes a phone call to the right agency when something just doesn't feel right.
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"Without the darkness, how would we see the light?" ~ Tuvok
Editor for Television Category