A agent with black sunglasses and a national security letter is a very different case from cops going on a fishing trip. It's a bad idea to ignore the former. B

A agent with black sunglasses and a national security letter is a very different case from cops going on a fishing trip. It's a bad idea to ignore the former. But there is probably (IANAL) no requirement to answer to queries of the latter, unless they also have a court order.