The case was shrouded in secrecy.
The man, whose identity and nationality have not been disclosed, was arrested on Friday. A court in Oslo on Sunday ordered that he be held in pre-trial custody for four weeks, on suspicion of espionage and intelligence operations against the NATO-member Nordic country.
In an email to The Associated Press, a prosecutor for Norway’s domestic security agency, known by its acronym PST, said the investigation was in “a critical and initial phase” and would take time.
During the arrest, police seized from the man a number of data-carrying electronic devices. The suspect is a student — though not enrolled in an educational institution in Norway — and has been living in Norway for a relatively short time, Norwegian media said.
Norwegian broadcaster NRK said the suspect had allegedly been caught conducting illegal signal surveillance in a rental car near the Norwegian prime minister’s office and the defense ministry.
The suspect, who authorities say was not operating alone, was banned from receiving letters and visits. According to prosecutor Thomas Blom, the suspect “has not yet wanted to be questioned.”
Blom declined to comment further.
In previous assessments, the security agency has singled out Russia, China and North Korea as states that pose a significant intelligence threat to Norway, a nation of 5.4 million people.
In October, Norway detained a man who had entered the country as a Brazilian citizen but is suspected of being a Russian spy. He was detained in the Arctic city of Tromso, where he worked at the Arctic University of Norway.
Norwegian media have said the man called himself Jose Assis Giammaria. Norwegian authorities said he was 44, born in Russia in 1978 and was likely named Mikhail Mikushin.