A nonprofit organization called The Internet Archive has prevailed in a long battle to resist the FBI’s demand for information about one of its users, subjecting the Bureau’s “National Security Letters” to an unprecedented level of scrutiny.

The scrutiny is unprecedented because National Security Letters often include an extremely powerful, effectively permanent gag order that prevents the recipient from discussing the FBI’s demand with anyone except their legal counsel. When the FBI finally rescinded its NSL and unsealed the relevant court records, a window into the process was opened for privacy advocates. It’s now possible to read a partially redacted copy of the famously secretive National Security Letter itself.

The letter contains some problematic wording, as the Electronic Frontier Foundation explains:

The NSL issued to the Archive said the library had the right to “make an annual challenge to the nondisclosure requirement.” But in 2015, Congress updated the law to allow for more than one request a year, so that communications providers could speak out about their experience without unneeded delay. Represented by the Electronic Frontier Foundation (EFF), the Archive informed the FBI that it did not have the information the agency was seeking and pointed out the legal error. The FBI agreed to drop the gag order in this case and allow the publication of the NSL.

“The free flow of information is at the heart of the Internet Archive’s work, but by using national security letters in conjunction with unconstitutional gag orders, the FBI is trying to keep us all in the dark,” said Brewster Kahle, founder and digital librarian of the Internet Archive. “Here, it’s even worse: that secrecy helped conceal that the FBI was giving all NSL recipients bad information about their rights. So we especially wanted to make this NSL public to give libraries and other institutions more information and help them protect their users from any improper FBI requests.”

“The opaque NSL process – including the lack of oversight by a court – makes it very vulnerable to errors of law.  Add to that the routine use of gags and enforced secrecy, and those errors become difficult to find and correct,” said Electronic Frontier Foundation Staff Attorney Andrew Crocker, who went on to thank the Internet Archive for “standing up to the FBI and shining some light on this error.”

The EFF said it is involved in three other challenges to National Security Letters and their attendant gag orders, prominently including CREDO Mobile. It also revealed that the Internet Archive successfully resisted a previous NSL in 2007.

The Intercept notes that the Internet Archive, whose subscribers can “use their accounts to upload contributions of books, music and other digital material or to comment on material others have uploaded,” did not actually have any records for a subscriber matching the FBI’s request.

Until now, there have been few challenges to National Security Letters or nondisclosure requirements; The Intercept describes a few others, and quotes former Attorney General Eric Holder telling Congress that only four challenges were raised against 50,000 National Security Letters issued between 2008 and 2010.

Privacy advocates argue that one reason for the paucity of challenges is that the actual subjects of the NSLs don’t know about them, due to the gag orders. The subscriber to an Internet service has no idea the FBI has demanded his or her data from the service provider. Of course, supporters of the NSL instrument argue that such secrecy is vital, to prevent suspected terrorists from knowing they are under surveillance.

The 2015 adjustments Congress made to the enabling legislation, and which were not properly explained in the letter sent to the Internet Archive, were prompted by serious Constitutional concerns about the gag orders. The more aggressive critics of National Security Letters say it was no innocent clerical error that left the revised rules for challenging non-disclosure out of the standard NSL text.

FiveThirtyEight.com has filed a Freedom of Information Act request to obtain the exact number of National Security Letters sent with incorrect nondisclosure requirements by the FBI.