Jonathan Toebbe, 42, who worked as a nuclear engineer for the US Navy, and his wife Diana, 45, were arrested by Federal Bureau of Investigation agents on Saturday and charged with violating the Atomic Energy Act in a criminal complaint, the Justice Department said in a statement.

For nearly a year, the couple "sold information known as restricted data concerning the design of nuclear powered warships to a person they believed was a representative of a foreign power," the statement said, adding that the recipient of the information was in fact an undercover FBI agent.

US nuclear-powered submarines were at the center of a major diplomatic crisis between Paris and Washington last month, when Australia scrapped a large contract to buy submarines from France.

Toebbe, who was based in Annapolis, Maryland and had access to restricted data on nuclear-powered warships through his work, collaborated with his wife to exchange data on submarine nuclear reactors for around $100,000 in cryptocurrency, the complaint affidavit alleges.

When Toebbe first tried to establish contact with the foreign government -- which was not identified in the affidavit -- he wrote, "I apologize for this poor translation into your language. Please forward this letter to your military intelligence agency," according to the complaint.

"I believe this information will be of great value to your nation. This is not a hoax."

The affidavit says Toebbe in April 2020 mailed a package to the foreign government "containing a sample of restricted data and instructions for establishing a covert relationship," according to the statement.

According to the complaint, the FBI intercepted the package, and the undercover agent posed as a representative of the foreign government in order to establish a relationship with the couple.

The agent sent Jonathan Toebbe, who used the pseudonym "Alice," an email offering him a gift as thanks for the data. But Toebbe responded with caution, asking instead to set up a "dead drop" location and to be paid in cryptocurrency.

The couple in following months transferred multiple encrypted SD (secure digital) cards of data to the agent. In the first dead drop, "the SD card was wrapped in plastic and placed between two slices of bread on a half a peanut butter sandwich," the agent said in the complaint.

Other SD cards were hidden in a chewing gum package and a sealed Band-Aid wrapper, with the Band-Aid still inside.

The couple was arrested Saturday after leaving another SD card at a dead drop location.

US Attorney General Merrick Garland praised the agencies involved for "thwarting the plot" and "taking this first step in bringing the perpetrators to justice," according to the statement.

The Department of Defense declined to comment when contacted by AFP.

The couple is due to appear in West Virginia federal court on October 12.

US nuclear-powered submarines have been the subject of recent attention when Australia and Britain in September unveiled a new defense contract for such subs, negotiated secretly over several months.

In order to join the pact, Australia tore up a landmark multimillion-dollar contract with France, sparking an international row that saw Paris recall its ambassador to Canberra.