The FBI has sent out tens of thousands of "national security letters" (NSLs) in the years following the 9/11 attacks, under expanded authority given to the bureau by the PATRIOT Act. The letters nearly always come with a "gag order" preventing the recipient from even revealing the existence of the letter.

A few weeks ago, the FBI's practice suffered its first major setback. A San Francisco federal judge ruled that sending letters with a gag order is unconstitutional because the gag orders take away recipients' free speech rights. That case was brought by the Electronic Frontier Foundation, representing an unnamed telecom company.

Now, Google has stepped forward to become the second company to challenge the letters, according to Bloomberg. The company's case is in front of the same judge who ruled the letters unconstitutional, US District Judge Susan Ilston. She ordered the FBI to stop sending the letters on March 14, but she stayed her order for 90 days so the agency could appeal.

Last week, the search giant filed court papers in a case called In Re Google Inc. Petition to set aside Legal Process. The court documents are mostly sealed, but one unsealed document in the case indicates that Google is challenging Section 2709 of Title 18, which deals with NSLs. That suggests Google is pushing back against a government demand for user information.

The government has issued more than 200,000 NSLs since 2000, according to Matt Zimmerman, the EFF attorney who successfully challenged NSLs in Ilston's court in the telecom case. Zimmerman told Bloomberg that Google's pushback is a good thing. “So far no one has really stood up for their users” among large Internet service providers, Zimmerman said.

Both Google and the FBI told Bloomberg they wouldn't comment on the case.

Last month, Google released its first "transparency report" regarding its receipt of NSLs, but the data it was allowed to release was almost comically vague. The company only said it had received between zero and 999 NSLs per year between 2009 and 2012.