A US nuclear engineer and his wife appeared in court Tuesday days after their arrest for trying to sell submarine secrets for $5 million, as speculation persisted on which country was the targeted buyer.

Jonathan Toebbe and his wife Diana Toebbe appeared separately in federal court in West Virginia dressed in orange prison garb. A judge ordered them to be assigned public defenders, indicating they did not have enough money to hire their own.

Both are charged with conspiracy to hand over highly classified technology on the navy's most advanced nuclear-powered, cruise missile-launching submarines to an unnamed foreign power.

A criminal complaint unveiled Sunday does not say which country they sought to sell the information to but suggests that it could be a US ally, disclosing that the country informed the US Federal Bureau of Investigation of the Toebbes' overture last December.

It also does not provide any motive for the couple.

Jonathan Toebbe, 42, was a former officer in the navy with expertise in the nuclear propulsion of submarines.

After leaving the navy he worked as a civilian contractor assigned to a research laboratory in Pittsburgh, Pennsylvania that designs and develops nuclear power for the navy.

Diane Toebbe, 45, meanwhile was a teacher at a well-regarded private school, the Key School, in Annapolis, Maryland where the couple owned a home.

The complaint gave tantalizing, spy-novel-like details of the case, involving dead-drops, cryptocurrency payments, and signals from an embassy building in Washington.

After an initial good-faith payment, the FBI lured them over encrypted communications to deliver an SD card loaded with secret information on submarine technology.

The card was tucked into a peanut butter sandwich and left at a location in West Virginia about 100 miles (160 kilometers) from Annapolis.

The second dead drop, in July, involved hiding an SD card inside a wrapper containing a Band-Aid and left in a plastic baggie at a site in south-central Pennsylvania.

In a message to the "buyers" -- undercover FBI agents -- Toebbe indicated that he had been considering his actions for several years and was now happy to work with "a reliable professional partner."

He also wrote that he had divided all the data he had collected into 51 "packages" of information. He wanted $100,000 for each, to be delivered in batches over an undetermined time period.

In a third drop in August, Toebbe left more submarine data and a note saying that if he ran into trouble he hoped the foreign country would help "extract" him and his family.

"We have passports and cash set aside for this purpose," Toebbe wrote.

He added his gratitude for the "partnership."

"One day, when it is safe, perhaps two old friends will have a chance to stumble into each other at a cafe, share a bottle of wine and laugh over stories of their shared exploits," he wrote.

The couple was arrested on Saturday and face possible imprisonment for life on each of the charges.

Experts speculated on the identity of the country which alerted the FBI of the data offer in December, nearly nine months after the Toebbes first mailed their offer to the country's military intelligence.

One of their communications indicated that English might not be the country's native tongue, and others suggest that the country's navy is familiar with nuclear propulsion technology.

Besides the United States, Russia, China, Britain, France, and India operate nuclear-powered naval vessels.