"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," Lavabit LLC owner Ladar Levison wrote in a letter posted on the Texas-based company's website Thursday.

Levison said he has 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 Snowden went public as the source of media reports detailing secret electronic spying operations by the US National Security Agency.

"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," Levison wrote.

The US Department of Justice had no immediate comment.

Later on Thursday, 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, on Twitter and in a blog post said Silent Circle had ended Silent Mail.

"We see the writing on 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," Callas wrote on a blog for customers.

Silent Circle, co-founded by PGP cryptography inventor Phil Zimmermann, will continue to offer secure texting and secure phone calls, but email is harder to keep truly private, Callas wrote. He and company representatives didn't immediately respond to interview requests.

At a Moscow news conference four weeks ago, a Human Rights Watch representative said she had been contacted by Snowden from a Lavabit email address, according to news website GlobalPost.com.

Use of effective encryption by regular email users is rare.