A suspected spy and accomplice were arrested on Tuesday in an early-morning raid in Sweden.
Witnesses described the dramatic dawn operation with Black Hawk helicopters descending on a Stockholm suburb.
One of the pair was arrested on suspicion of "gross unlawful intelligence activities" against Sweden and another country.
The second is suspected of aiding the espionage, while a third person was taken in for questioning.
The newspaper Aftonbladet said the two suspects were a married couple who had moved to Sweden from Russia.
Officials did not confirm which country they suspected was behind the espionage.
Police spokesman Stefan Hector said authorities had to move quickly to preserve evidence.
"You should not be able to flush anything down the toilet or delete any data," he told broadcaster SVT.
Security services spoke of a long investigation culminating in a raid involving Sweden's armed forces.
Prosecutors have until Friday to decide on possible charges.
Few other details were given but Swedish media said the alleged espionage could have been going on for 10 years.
Authorities said the case had no connection to charges revealed on November 11 against two suspected Russian spies.
One of those suspects was a former employee of Sweden's armed forces and security services.
Western countries have reported an increase in foreign intelligence activity during Russia's invasion of Ukraine.