A US navy nuclear engineer with access to military secrets has been charged with trying to pass information about the design of American nuclear-powered submarines to someone he thought was a representative of a foreign government but who turned out to be an undercover FBI agent, the Justice Department said.

In a criminal complaint detailing espionage-related charges against Jonathan Toebbe, the government said he sold information for nearly the past year to a contact he believed represented a foreign power.

That country was not named in the court documents.

Toebbe, 42, was arrested in West Virginia on Saturday along with his 45-year-old wife Diana after he had placed a removable memory card at a pre-arranged “dead drop” in Jefferson County, according to the Justice Department.

The Toebbes are from Annapolis, Maryland.

The FBI says the scheme began in April 2020 when Jonathan Toebbe sent a package of navy documents to a foreign government and said he was interested in selling operations manuals, performance reports and other sensitive information.

Authorities say he also provided instructions for how to conduct the furtive relationship, with a letter that said: “I apologise for this poor translation into your language. Please forward this letter to your military intelligence agency. I believe this information will be of great value to your nation. This is not a hoax.”

The FBI’s legal office in the foreign country received the package, which had a return address of Pittsburgh, last December.

That led to a months-long undercover operation in which an agent posing as a representative of the foreign government offered to pay thousands of dollars in cryptocurrency for the information Toebbe was offering.

In June, the FBI says, the undercover agent sent 10,000 dollars (£7,343) in cryptocurrency to Toebbe, describing it as a sign of good faith and trust.

The following week, FBI agents watched as the Toebbes arrived at an agreed-upon location in West Virginia for the exchange, with Diana Toebbe appearing to serve as a lookout for her husband during the dead-drop operation, according to the complaint.

The FBI recovered a blue SD card wrapped in plastic and placed between two slices of bread on a peanut butter sandwich, the complaint says.

The FBI paid Toebbe 20,000 dollars (£14,686) for the transaction and provided the contents of the SD card to a navy subject matter expert, who determined that the records included design elements and performance characteristics of Virginia-class submarine reactors, the Justice Department said.

Those submarines are sophisticated, nuclear-powered “cruise missile fast-attack submarines”, according to the complaint.

The complaint said the SD card also included a typed message that said, in part: “I hope your experts are very happy with the sample provided and I understand the importance of a small exchange to grow our trust.”

The FBI conducted similar dead-drop exchanges over the next several months, including an August one in Virginia in which Toebbe was paid 70,000 dollars (£51,400) and concealed an SD card in a chewing gum package, the complaint says.

The complaint alleges violations of the Atomic Energy Act, which restricts the disclosure of information related to atomic weapons or nuclear materials.

The Toebbes are expected to have their initial court appearances on Tuesday in Martinsburg, West Virginia.

Jonathan Toebbe has worked for the US government since 2012, holding a top-secret security clearance and specialising in naval nuclear propulsion, the FBI says.

He has also been assigned to a laboratory in the Pittsburgh area that officials say works on nuclear power for the US navy.