A scientist and his wife who both once worked at Los Alamos National Laboratory were arrested today after an FBI sting operation and charged with offering to help develop a nuclear weapon for Venezuela.
They were accused of dealing with an FBI undercover agent posing as a Venezuelan agent.
The government did not allege that Venezuela or anyone working for it sought US secrets.
Pedro Leonardo Mascheroni, 75, and Marjorie Roxby Mascheroni, 67, a US citizen, were arrested yesterday.
They appeared in federal court in Albuquerque, where Mascheroni, a naturalised US citizen from Argentina, was ordered held pending another hearing on Monday.
His wife was released under strict conditions.
Kenneth Gonzales, US attorney for New Mexico, said the indictment does not allege the government of Venezuela or anyone acting on its behalf sought or was passed any classified information.
It has been known for about a year that Mascheroni was under investigation - the FBI last October seized computers, letters, photographs, books and cell phones from the couple's Los Alamos home.
At the time he said he believed the US government was wrongly targeting him as a spy. He has denied the accusation.
Mascheroni said that he approached Venezuela after the United States rejected his theories that a hydrogen-fluoride laser could produce nuclear energy.
According to the 22-count indictment, Mascheroni told the undercover agent he could help Venezuela develop a nuclear bomb within 10 years and that Venezuela would use a secret, underground nuclear reactor to produce and enrich plutonium, and an open, aboveground reactor to produce nuclear energy.
If convicted, the Mascheronis face up to life in prison.
Many previous FBI spy sting cases have begun this way: US intelligence learns, often by electronic surveillance, that someone in this country is trying to contact a foreign power to offer their services or US secrets.
Then the FBI has an undercover agent pose as a representative of that country to respond favourably, cultivate a relationship and see what, if any, secrets the person tries to pass or sell.
Mascheroni worked in the nuclear weapons design division at the Los Alamos lab from 1979 until he was laid off in 1988. His wife, a technical writer, worked there between 1981 and 2010.
He told AP last year he was motivated by his belief in cleaner, less expensive and more reliable nuclear weapons and power. He began approaching other countries after his ideas were rejected by the lab and, later, congressional staffers.
The criminal charges allege Mascheroni delivered to a post office box in November 2008 a disk with a coded 132-page document on it that contained "restricted data" related to nuclear weapons.
Written by Mascheroni and edited by his wife, the document was entitled "A Deterrence Program for Venezuela" and laid out Mascheroni's nuclear weapons development program for Venezuela.
Mascheroni stated the information he was providing was worth millions of dollars, and that his fee for producing the document was 793,000 dollars (£515,000), the indictment alleges.