New Mexico: A scientist and his wife who both once worked at Los Alamos National Laboratory were arrested 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, but 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 on Friday, a day after they were indicted. They appeared in federal court in Albuquerque, where Mascheroni, a naturalised US citizen from Argentina, was ordered held pending another hearing on Monday morning. 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. Gonzales did not take questions after giving a brief statement to reporters.

It's 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. In an interview with The Associated Press at the time, he said he believed the US government was wrongly targeting him as a spy. He has denied the accusation.

Mascheroni said in the interview 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.

In July 2008, the undercover FBI agent provided Mascheroni with 12 questions purportedly from Venezuelan military and scientific personnel.

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, the indictment alleges.

Earlier in the investigation, Mascheroni allegedly asked the FBI agent about obtaining Venezuelan citizenship.

He told the undercover agent he should be addressed as "Luke," and he would set up an e-mail account solely to communicate with the undercover agent, according to the indictment.

Mascheroni used the account to communicate with the agent and to arrange for deliveries of materials at the post office box used as a dead-drop location, authorities say.

In June 2009, Mascheroni received another list of questions, again purportedly from Venezuelan officials, and $20,000 in cash from the FBI agent as a first payment.

The following month, Mascheroni delivered a disk that contained a 39-page document with answers to the questions. The document was allegedly written by Mascheroni, edited by his wife, and contained "restricted data" related to nuclear weapons.

Mascheroni allegedly wrote that the information he provided was classified and was based on his knowledge of US nuclear tests he had learned while working at Los Alamos. But the government said Mascheroni also wrote that he would state the document was based on open information found on the Internet if "our relationship/alliance does not work."

He told the AP last year he thought the Venezuelan government wanted him to produce a study on how to build a nuclear weapons program. In return, he asked for $800,000, which he said he planned to use for his scientific research on nuclear fusion in hopes of persuading Congress to take a look at his theories.

He said he received a formal request via e-mail from his Venezuelan contact in July 2008 to write the study. Mascheroni told AP he finished the study in November 2008 and, following directions, placed a CD containing only unclassified information available on the Internet - which he already had provided to congressional staffers - inside a post office box at the Albuquerque airport.

Later, he told AP, he received an e-mail telling him to return to the same post office box where he found a note saying there was $20,000 in $100 bills inside an envelope. He has said he never opened the envelope, and that FBI agents opened it when they searched his home.