Two encrypted email services have closed down for reasons linked to US intelligence leaker Edward Snowden.
Texas-based Lavabit service has shut down but said legal reasons prevented it explaining why.
Correspondents say Lavabit appears to have been in a legal battle to stop US officials accessing customer details.
In addition, secure communications firm Silent Circle has shut its email service because messages cannot be kept wholly secret.
Mr Snowden, a former contractor to the American National Security Agency (NSA), has admitted leaking information about widespread US surveillance on electronic communications to the media.
He fled the US - where he now faces espionage charges - and has been granted temporary asylum in Russia.
Lavabit came under scrutiny following reports that Mr Snowden was using the service while holed-up in Moscow airport.
"I have been forced to make a difficult decision: to become complicit in crimes against the American people, or walk away from nearly 10 years of hard work by shutting down Lavabit," Mr Levison wrote in a letter posted on the Lavabit website.
He said he had decided to "suspend operations" but was barred from discussing the events over the past six weeks that led to his decision.
"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.
Silent Circle said it shut down its email service for both technical and political reasons.
"Email as we know it... cannot be secure," wrote Jon Callas, co-founder and head of technology officer at Silent Circle, in a blogpost. "Email that uses standard Internet protocols cannot have the same security guarantees that real-time communications has."
By contrast, he said, the firm was keeping its secure voice and text services going because it had control over the infrastructure supporting them and could guarantee that messages were not intercepted or tampered with en route.
In addition, said Mr Callas, it was anticipation of future government calls to hand over customer details that prompted the Silent Mail shutdown.
"We see the writing (on) the wall, and we have decided that it is best for us to shut down Silent Mail now," he said. "We have not received subpoenas, warrants, security letters, or anything else by any government, and this is why we are acting now.
Speaking to the BBC, Silent Circle co-founder Phil Zimmermann said the service was closed because of Lavabit's action and because it feared it would be coerced into handing over keys that can unscramble messages.
In addition, he said, email was very hard to make secure. While the contents of messages can be scrambled little can be done about "metadata" which can give clues about who is talking to whom.
The US Department of Justice has so far not commented on the Lavabit closure.
Jennifer Granick, director of civil liberties at the Stanford Law School's Center for Internet and Society, said America's widespread surveillance could have far-reaching consequences for its technology industry.
"...the US government, in its rush to spy on everybody, may end up killing our most productive industry," she wrote in a blogpost. "Lavabit may just be the canary in the coal mine."