Lavabit, an email service offering encrypted communications that are supposedly impenetrable by third parties, shut down this week after it came to light that National Security Administration leaker Edward Snowden was using it to communicate from Moscow. Snowden’s Lavabit address was revealed by Tanya Lokshina, Deputy Director of Human Rights Watch’s Moscow office, in a Facebook post about an invitation she received to meet with the leaker while he was still holed up at Sheremetyevo Airport’s transit zone.

“I have been forced to make a difficult decision: to become complicit in crimes against the American people or walk away from nearly ten years of hard work by shutting down Lavabit. After significant soul searching, I have decided to suspend operations,” reads an open letter to users posted on the site by Lavabit creator Ladar Levison.

“I think it’s important to emphasize that this was a question of protecting everyone’s privacy, not that of a particular user,” Levison told TIME in a phone conversation Friday afternoon. “I’ve always been comfortable with responding to a properly executed court order, provided it was specific, and I was turning over the information that I had. But I believe it’s gone beyond that, and we’re at a situation now where I had to make this decision to protect the privacy of all my users, not just one or two.”

Lavabit had approximately 410,000 users when Levison pulled the plug. Thousands of supporters have donated $90,000 to his legal defense fund as of late Friday, and he said he’ll take the issue “all the way to the Supreme Court” if he’s able. Levison didn’t share what happened to him or his company that made him choose to shut Lavabit down — in fact, he told TIME he was legally barred from doing so. But his post offers some clues that can help us understand what probably happened.

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Levison said in his blog post that he’s fighting “for the Constitution in the Fourth Circuit Court of Appeals.” That means he’s not dealing with the Foreign Services Intelligence Act (FISA) Court, and that he may have received and is fighting against a National Security Letter, or NSL. The Federal Bureau of Investigation can issue NSLs without court approval in national security investigations to compel electronic communication providers to hand over a user’s data. That includes their name, address, length of service and other details, though not the content of their messages. They almost always come with a gag order preventing the recipient from telling the user or anyone else about the letter, which may explain Levison’s lack of detail. (Gag orders also come with records requests issued under Section 215 of the PATRIOT Act, which Levison may have received instead of an NSL.)

Should this be Levison’s first run-in with an NSL, it might also suggest the warning he left on his website: “This experience has taught me one very important lesson: without congressional action or a strong judicial precedent, I would strongly recommend against anyone trusting their private data to a company with physical ties to the United States,” he wrote.

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If Levison is indeed fighting against an NSL, he has plenty of company. Earlier this year, Google was revealed to be the company challenging 19 such letters in highly secretive proceedings at the U.S. District Court in San Francisco. (Even Google’s name was redacted from most court documents, but it was later found by media outlets in the initial filings.) Google lost that fight in late May, but U.S. District Judge Susan Illston hinted that Google could try again with arguments more specific to the precise NSLs in question.

In a separate lawsuit brought by the Electronic Frontier Foundation, the same judge struck down the law establishing NSLs, ruling their secrecy requirements violate the First Amendment. Judge Illston also struck down a rule prohibiting NSL recipients from challenging letters in court, but she gave the Obama administration 90 days to appeal, which it did in early May. The case has since moved to the Ninth Circuit Court of Appeals.

Lavabit, then, wasn’t forced to close simply for offering services to Snowden. Instead, Levison is convinced that the government’s ability to demand data about his users means he simply can’t offer the private email services that he advertises. Ultimately, he’s concluded that the only proper course of action was shutting down until that changes — if it ever does.

“This all happened so quickly and so suddenly that I haven’t really been able to make those plans,” Levison said when asked if he had other employment options lined up. “It’s really just been about reacting.”