An encrypted email service believed to have been used by US fugitive Edward Snowden shut down abruptly yesterday.
The closure appears to be part of a legal fight involving US government attempts to win access to customer information.
The owner of the Texas-based Lavabit LLC company said he had decided to "suspend operations" but was barred from discussing the events over the past six weeks that led to his decision.
That matches the period since Mr Snowden went public as the source of media reports detailing secret electronic spying operations by the US National Security Agency.
Company owner Ladar Levison said in a letter posted on his website: "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.
"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 added.
The US Department of Justice had no immediate comment.
Later, an executive with a better-known provider of secure email said his company had also shut down that service.
Jon Callas, co-founder of Silent Circle Inc, said on Twitter and in a blog post that Silent Circle had ended Silent Mail.
"We see the writing the wall, and we have decided that it is best for us to shut down Silent Mail now.
"We have not received subpoenas, warrants, security letters, or anything else by any government, and this is why we are acting now," Mr Callas wrote on a blog addressed to customers.
He said Silent Circle, co-founded by the PGP cryptography inventor Phil Zimmermann, will continue to offer secure texting and secure phone calls, but email is harder to keep truly private.
At a Moscow news conference last month, a Human Rights Watch representative said she had been contacted by Mr Snowden from a Lavabit email address, according to news website GlobalPost.com.
Mr Snowden has been charged with espionage but was granted asylum by Russia.
Some of Mr Snowden's leaked documents show that Google Inc, Microsoft Corp and other large providers have been compelled to help intelligence authorities gather email and other data on their users.
The big providers and other companies typically offer encryption but said they cooperate with legal requests, including those by intelligence officials.
Lavabit's statement suggested a gag order was in place, and lawyers said that could accompany any one of a wide range of demands for information.