NBC's Michael Isikoff reports that senior U.S. Attorney James Trump sent Lavabit founder Ladar Levison and his lawyer an apparent arrest threat when Levison shut down his private email service -- used by NSA leaker Edward Snowden -- rather than comply with a secret order to spy on his customers.

"I could be arrested for this action," Ladar Levison told NBC News about his decision to shut down his company, Lavabit LLC, in protest over a secret court order he had received from a federal court that is overseeing the investigation into Snowden.

Lavabit said he was barred by federal law from elaborating on the order or any of his communications with federal prosecutors. But a source familiar with the matter told NBC News that James Trump, a senior litigation counsel in the U.S. attorney’s office in Alexandria, Va., sent an email to Levison's lawyer last Thursday – the day Lavabit was shuttered -- stating that Levison may have "violated the court order," a statement that was interpreted as a possible threat to charge Levison with contempt of court.

Trump, who has been a lead attorney on high-profile leak investigations targeting former CIA officers John Kiriakou and Jeffrey Sterling, did not respond to a request for comment, nor did prosecutors in the U.S. Attorney’s Office, whose prosecutors have charged Snowden with violations of the Espionage Act. "We have no comment," said Andrew Ames, a spokesman for the Justice Department.

Levison, a 32-year-old entrepreneur who ran his company out of a Dallas apartment, said in a public statement last Thursday that he made "the difficult decision" to shut down Lavabit because he did not want "to become complicit in crimes against the American people."

Public statement issued by Levinson explaining his decision to shut down his company:

"I wish that I could legally share with you the events that led to my decision," Levinson wrote. "I cannot." Levison said he was barred from discussing the events over the past six weeks that led to his decision. It is believed to be a sealed subpoena or national security letter which demands that he cooperate with an investigation related to Snowden.

Levison plans to challenge the secret order in a federal appeals court. He told NBC that he has been “threatened with arrest multiple times over the past six weeks.”

He stated that he has complied with court orders for information on targeted customers in the past, but insists the latest order is vastly different in scope and scale.

Following Levison’s move to shutter Lavabit, encrypted internet service provider Silent Circle has followed suit. Other encryption services have suggested that they would do the same if put in a similar position by the US government.

Riseup email service issued a statement saying, “We would rather pull the plug than submit to repressive surveillance by our government, or any government.”

Encrypted chat client Cryptocat stated, “If we receive a surveillance or backdoor order that we are unable to legally fight, we will shut down Cryptocat rather than implement it.”

Levison began a legal defense fund which raised over $40,000 within hours of Lavabit’s shutdown. That number had jumped to $90,000 by the next day. Among those now backing him is former Texas congressman and Republican presidential candidate Ron Paul, who told NBC News on Tuesday that Levison's legal battle "should be in the interests of everybody who cares about liberty."