Wired:

The FBI has been issuing national security letters for decades. The controversial subpoenas, which allow the feds to obtain customer records and transaction data from internet service providers and other companies without a court order, come with a perpetual gag order that prevents recipients from disclosing that they’ve received an NSL.

Only a small handful of recipients have ever publicly disclosed that they got one from the government, and only after lengthy court battles challenging the subpoenas. But today, Yahoo became the first company to go public about NSLs it has received without needing to duke it out with the feds in court.

Here’s a link to the redacted letters if you care to have a peek. I found them pretty fascinating.