Konstantin Antonets and Antonina Zimina were detained in July 2018 in Russia's Baltic Sea exclave of Kaliningrad and went on trial behind closed doors in May this year. The couple maintains they are innocent.

Zimina's father, Konstantin Zimin, told RFE/RL on July 16 that the couple had been additionally charged with travelling to Latvia, where they allegedly passed classified information to a Latvian intelligence operative.

"The former economy minister of the Kaliningrad region, Ninel Salagaeva, who is currently a deputy governor of the [western] Pskov region, testified that Konstantin [Antonets] had worked with some classified documents when he was employed at [the Kaliningrad region's] government and three or four years later, he and Antonina [Zimina] took them out of the country," Zimin said.

The Znak.com news website says the documents related to the new charge might be linked to information about the allocation of finances to the Kaliningrad region to support local residents and businesses.

Zimina's parents told RFE/RL in February that the Federal Security Service (FSB) suspected their daughter and her husband of providing the intelligence agency in the Baltic state of Latvia with information about a Kaliningrad FSB officer.

According to them, among the guests who attended the couple’s wedding in April 2015 was an FSB counterintelligence officer, Maksim Denisenko, who was their daughter's friend and former university classmate.

At the wedding, Denisenko spoke openly about his employer, offered "assistance if need be," and handed out business cards and posed for photographs with guests, Zimina's parents said.

The parents also told RFE/RL that their daughter's friends from Latvia and Lithuania also attended the wedding.

The daily Kommersant reported in February that some videos and photographs from the wedding were later published on social media, and eventually included in an unspecified Baltic television program, prompting the FSB to open a criminal investigation.

In recent years, the number of cases of alleged high treason has increased dramatically in Russia.

The latest high-profile high treason case involves Ivan Safronov, a journalist and an aide to the Russian Roskosmos space agency chief, Dmitry Rogozin.

Safronov was arrested on July 7 and later charged with passing classified materials to the Czech Republic.