COPENHAGEN, Denmark (AP) — Norway’s domestic security agency said Tuesday that Russian hackers linked to Russia’s military intelligence service GRU are “likely” behind a cyberbreach earlier this year of the Norwegian parliament's email system.

The security agency said the email system handles information "of great interest to several foreign states’ intelligence services.”

The agency, known by its acronym PST, concluded that the network operation that targeted the Storting assembly on Aug. 24 was “part of a larger campaign nationally and internationally, which has been going on at least since 2019.” PST didn't elaborate.

The intelligence agency said that “it is likely" that the operation was carried out by the hacker group known as APT28, or Fancy Bear. "This actor is linked to Russia’s military intelligence service GRU, more specifically their 85th Special Services Center (GTsSS),” it said in a statement.

The Russian Embassy in Norway said in a statement that the Norwegian probe produced no evidence of Russian involvement, and said the claim was “unacceptable.”

PST said it had terminated the investigation as it did not provide “sufficient information” for an indictment.

The agency echoed the Norwegian parliament's own investigation into the breach in saying it found vulnerabilities such as insecure passwords. It said that common security mechanisms such as two-factor authentication, could stymie such cyberattacks.

Foreign Minister Ine Eriksen Soereide said Oct. 14 that “it is our assessment that Russia is behind this activity” and called it “a serious incident that affects our most important democratic institution.”

Russia's diplomatic mission pointed out that Russian authorities had notified Oslo about malicious online activities originating in Norway on six occasions in 2019 and four times this year, adding that Russia hasn’t received any such notices from Norway.

Russian officials have repeatedly denied any involvement by Moscow in any hacking attacks. They have similarly dismissed charges of Russian meddling in the 2016 U.S. presidential election and alleged cyberattacks on other Western nations and institutions.

Norway and Russia that share a nearly 200-kilometer (124-mile) land border, have been at odds in recent months.

This summer, both Moscow and Oslo expelled one diplomat each after a Norwegian citizen was jailed in Norway on accusations of industrial spying for Russia. The suspected spy who denies wrongdoing, was arrested in an Oslo restaurant on Aug. 15.

Last year, a retired Norwegian border inspector was sentenced in Russia to 14 years in prison for espionage. He was arrested in Moscow in December 2017 and accused of collecting information about Russian nuclear submarines for Norwegian intelligence.

Later, Frode Berg was part of a three-way spy swap when he and two Lithuanians were freed in exchange for two Russians who had been in prison in Lithuania.