Five people were initially arrested and three remain jailed, the BBC said. Two suspects were released and police said Tuesday eight more possible suspects were being sought.
The paintings worth millions were stolen off the wall in an armed robbery at Oslo's Munch Museum last year. Neither work had been recovered as of Tuesday.
"We're still hopeful, but we are no longer confident (the paintings will be recovered). The chances of finding the paintings diminish with time and already a lot of a time has passed," Museum spokesman Jorunn Christoffersen told the BBC.
There was concern in the art world that the paintings were destroyed after the thieves realized they were too well known to sell.