The Internet Archive, a nonprofit organization that has built a digital library and maintains an archive of web pages on the internet, revealed on Thursday that it had received a National Security Letter from the FBI demanding information about the services the library provided to a possible user. National Security Letters such as this one have been criticized by civil liberties groups in part because they can include a nondisclosure requirement or “gag order” that prevents recipients from revealing anything about the letters — including the fact that they received them.

This letter was particularly troublesome to privacy advocates because it contained misinformation about the rights of a letter recipient to challenge the nondisclosure requirement. The letter stated that the Internet Archive could “make an annual challenge to the nondisclosure requirement.” The Electronic Frontier Foundation, an advocacy organization that is legally representing the Internet Archive, pointed out in a press release that the passage of the USA Freedom Act in June of 2015 changed the law to allow letter recipients to challenge the National Security Letter at any time, not just once annually. In response to the EFF’s claim, the FBI withdrew its National Security Letter, allowed the Internet Archive to publish a redacted version of the letter containing the error and promised to correct the mistake by informing everyone else who got the same erroneous language.

National Security Letters have become a common mechanism for the government to compel companies to provide customer information to the FBI. Although these letters have been around since the 1970s, the USA Patriot Act, passed in 2001, expanded the power of these letters and made it easier for the FBI to issue them. Since then, efforts by companies, lawyers and internet activists including the EFF have slowly chipped away at some of the secrecy of these letters. The Internet Archive is one of the few organizations to have successfully challenged, and subsequently made public, a National Security Letter.

The FBI sent out over 48,000 requests for information using National Security Letters last year, according to the Office of the Director of National Intelligence. The Intercept, an online news site, on Thursday reported that the FBI issued fewer than 10,000 National Security Letters last year. That figure, however, appears to refer to the number of requests for information concerning only U.S. persons and to exclude requests demanding only “subscriber information” such as names and addresses of a site’s customers or registered users. Betsy Reed, The Intercept’s editor-in-chief, declined to comment.

Regardless of the exact numbers, tens of thousands of information requests have probably been sent since Congress made it easier to challenge gag orders in security letters. An FBI spokesman declined to provide the number of letters sent with the same erroneous information as the one sent to the Internet Archive; FiveThirtyEight has filed a Freedom of Information Act request seeking that information.

Carl Bialik contributed to this article.