Pål K. Lønseth, head of Økokrim – the economic crime unit investigating Jagland – said: “We consider there are reasonable grounds for investigation, given that he held the positions of chair of the Nobel Committee and Secretary General of the Council of Europe during the period covered by the released documents.
“Among other things, Økokrim will investigate whether gifts, travel and loans were received in connection with his position.”
The police added that further details would be given “at a later stage”.
Norwegian Foreign Minister Espen Barth Eide said it was “important that the facts of this case are brought to light”.
He added that Oslo would ask the Council of Europe for Jagland’s immunity to be revoked.
Jagland’s lawyer Anders Brosveet welcomed the opening of the investigation.
“Based on what we have found so far, we are confident of the outcome,” he told Reuters news agency.
Separately, Brosveet told Norway’s VG news site that he and his client would submit “the key findings and documentation we have to Økokrim as soon as practically possible”.
Jagland was Norway’s prime minister in 1996-97, and the secretary general of the Council of Europe in 2009-19.
He also chaired the Norwegian Nobel Committee in 2009-15.