The target of a federal investigation that set off a more than decadelong battle over secret subpoenas called national security letters was a Muslim prison reform advocate the FBI wanted to become an informant.

Nick Merrill, who fought to make the information public, revealed that information for the first time at a hacker conference in New York City.

Merrill was the head of an internet hosting company when the controversy began. He had launched a small New York-based internet service provider called Calyx Internet Access in the 1990s, and he also consulted on digital security.

In 2004, the FBI sent him a national security letter demanding extensive records on one of his customers.

National security letters are secret subpoenas the FBI can send to internet and technology companies to demand various types of records about their customers’ online behaviors without ever getting a court order. In Merrill’s case, that request was particularly broad — for browsing records, email address information, billing information, and more.

In response, Merrill launched a court battle challenging the constitutionality of the letter itself, and then, when the FBI withdrew it, to free himself from the gag order forbidding him from ever speaking about it.

Speaking during the Hackers of Planet Earth, or HOPE, conference last week, Merrill shared previously nonpublic information about the nearly 12 years he spent trying to challenge the FBI’s request and then trying to lift the gag order placed on him after it was withdrawn. He disclosed additional details to The Intercept following his talk.

Merrill told The Intercept that the target of the national security letter was someone whose website he was hosting. He said the target was Muslim, but gave no further details, to protect his identity. He did not explain why the FBI considered this a national security matter.

Merrill’s national security letter was issued before President George W. Bush’s Department of Justice told the FBI it didn’t have the power to ask for such extensive records without consulting a judge — though as The Intercept reported in June, it’s not clear the agency ever actually stopped asking for such records.

Merrill said that as far as he knows, the target of the FBI investigation was never charged with a crime and turned over his hard drives voluntarily. The man had trouble finding jobs and boarding airplanes during the investigation, according to Merrill’s presentation.

Merrill said he maintained contact with the man for the entire 12 years, walking on eggshells when it came to the case and what he could share. One time, he told The Intercept, the target even sent him a photo of something he found on the bottom of his car — a device Merrill says a friend told him was a military-grade tracker.

Very little description about the target is included in the court documents that are public. Merrill wouldn’t provide his name. Merrill said the man has refused every interview request in the past.

The case reflects how little is known about the actual uses of national security letters — either about the targets or what information the FBI has sought.

“Part of the problem is that NSLs are secret and since they almost never result in prosecution, targets almost never get notice (which means we never find out who gets targeted),” Robyn Greene, surveillance policy counsel for the Open Technology Institute, wrote in an email to The Intercept. The secret requests “historically have been used for bulk collection.”

The FBI has been criticized by the Department of Justice inspector general in multiple reports for not following the rules when it comes to filing national security letters — sending them in bulk and without any real evidence of a crime. The Foreign Intelligence Surveillance Court in 2006 rejected an FBI request to obtain records because it “implicated the target’s First Amendment rights” — but the FBI went ahead and asked for the information anyway using a national security letter. The criticism has since led to reform, but the real impact of any changes is still secret.

The FBI declined to comment on Merrill’s new revelations.