The statement posted online by Lavabit owner Ladar Levison hinted that the Dallas-based company had been forbidden from revealing what was going on.
"I wish that I could legally share with you the events that led to my decision," Levison's statement said. "As things currently stand, I cannot share my experiences over the last six weeks, even though I have twice made the appropriate requests."
The post didn't name Snowden or refer to any particular investigation, but the statement's timing - and other material in the public domain - suggest that Lavabit shut down in protest at the U.S. government's pursuit of the 30-year-old leaker, whose disclosures have blown the lid off the NSA's secret domestic surveillance.
For example, Russian human rights campaigner Tanya Lokshina said in a Facebook post hours before she met Snowden at a Moscow airport last month that the leaker had contacted her using a Lavabit email address. And an online database hosted by the Massachusetts Institute of Technology showed that someone going by the name of "Ed Snowden" registered three addresses with Lavabit over the past four years.
Lavabit might have been attractive to Snowden because the company advertised itself as a secure, privacy-conscious alternative to webmail services operated by Yahoo and Google. The company's promotional material - it has since been pulled from the Internet - said Lavabit's system was specifically designed to resist secret requests from U.S. law enforcement.
Levison's statement said the firm had launched a legal defense fund and was preparing to go to court to "resurrect Lavabit as an American company."
Attempts to reach Levison weren't immediately successful.