Last year, when a telecommunications company received an ultra-secret demand letter from the FBI seeking information about a customer or customers, the telecom took an extraordinary step — it challenged the underlying authority of the FBI’s National Security Letter, as well as the legitimacy of the gag order that came with it.

Both challenges are allowed under a federal law that governs NSLs, a power greatly expanded under the Patriot Act that allows the government to get detailed information on Americans’ finances and communications without oversight from a judge. The FBI has issued hundreds of thousands of NSLs and been reprimanded for abusing them — though almost none of the requests have been challenged by the recipients.

After the telecom challenged its NSL last year, the Justice Department took its own extraordinary measure: It sued the company, arguing in court documents that the company was violating the law by challenging its authority.

That’s a pretty intense charge, according to Matt Zimmerman, an attorney for the Electronic Frontier Foundation, which is representing the anonymous telecom.

“It’s a huge deal to say you are in violation of federal law having to do with a national security investigation,” says Zimmerman. “That is extraordinarily aggressive from my standpoint. They’re saying you are violating the law by challenging our authority here.”