Two Swedish brothers, one a former intelligence official, went on trial in Stockholm on Friday accused of "aggravated espionage" for allegedly spying for Russia's GRU military intelligence service between 2011 and 2021.
"This case is unique in many ways ... We haven't had a trial like this in more than 20 years," prosecutor Mats Ljungqvist told the court in his opening statement.
He said the information obtained, transmitted, and divulged was "extremely sensitive material."
His co-prosecutor Per Lindqvist said it could be "detrimental to Sweden's national security."
Defendants Payam and Peyman Kia risk life sentences if found guilty. Most of the trial will be held behind closed doors.
"The court will have insight into material that very few in this country have seen or have access to," Ljungqvist said.
On Friday, prosecutors made brief introductory statements before the judge ordered reporters out of the courtroom.
Payam Kia is aged 35 and his brother is 42, according to the charge sheet. They are of Iranian origin, according to Swedish media reports.
Peyman Kia, who appeared calm in court dressed in a dark suit and tie, has served in Sweden's intelligence service Sapo and intelligence units in the Swedish army.
According to Sweden's newspaper of record, Dagens Nyheter, he at one point worked for the Office for Special Information Gathering (KSI), the most secret section of the military secret service.
He is accused of illegally acquiring information during his employment with Sapo and the armed forces
Payam Kia is accused of "participating in the planning of the deed and handling contacts with Russia and the GRU, including the handover of information and receiving compensation."
Bearded and dressed in the Swedish jail system's green overalls, he hid his face as he entered the courtroom with his lawyer.
Lawyers for the pair have been tight-lipped about the case. They told the court on Friday that their clients denied the charges.
The prosecutors requested that much of the material in the case be classified even after the end of the trial, due to its sensitive nature.
The names of several witnesses, including those working for the Swedish military and security police and who have access to vast amounts of classified information, will also be kept secret.
The case is expected to continue until Dec. 12.