Jonathan Toebbe, 43, pleaded guilty before a federal judge, more than four months after being arrested with his wife Diana Toebbe.

In exchange for his guilty plea, he is expected to be sentenced to between 12.5 to 17.5 years in prison.

His wife, a teacher, has so far maintained her innocence and sought release to care for their two teenage children. But her husband's plea deal also incriminates her.

"Diana Toebbe knowingly and voluntarily joined the conspiracy to communicate Restricted Data to another person with the intent to secure an advantage to a foreign nation and committed multiple overt acts in furtherance of the conspiracy, including acting as a lookout while Mr. Toebbe serviced three dead drops," the document said.

The court documents did not however say which country the couple tried to sell their information to.

The documents imply it was a US ally whose principal language is not English.

US nuclear submarines were at the center of a heated diplomatic crisis last September, when Australia canceled a mega-deal with France to announce a strategic partnership with the United States and Britain.

The Justice Department said in its statement that Toebbe had been working since 2012 on the design of the reactors for Virginia-class submarines, the latest generation of attack submersibles in the US fleet.

In April 2020, he had sent a package to a foreign country with an initial set of documents and instructions for establishing contact via a return address in Pittsburgh, Pennsylvania. The package contained "a sample of Restricted Data and instructions for establishing a covert relationship to purchase additional Restricted Data," the Justice Department said.

"Toebbe began corresponding via encrypted email with an individual whom he believed to be a representative of the foreign government. The individual was really an undercover FBI agent," the DoJ said.

Over several months, Toebbe received increasing payments in cryptocurrency for tens of thousands of dollars, and hid SD cards loaded with stolen secrets inside a peanut butter sandwich, in a packet of chewing gum and in a Band-Aid wrapper.

According to court documents, the country that Toebbe believed he was selling the secrets to was cooperating with the FBI, to the point of placing a flag in its Washington embassy to gain Toebbe's confidence.