"The clear main scenario" is that a state-sponsored group is behind the explosions, Ljungqvist said.
The prosecutor also did not rule out the participation of an independent group, He, however, noted that the explosives used during the attack are not widely used.
At the same time, Ljungqvist highlighted that it is still unclear who exactly is behind the Nord Stream gas pipelines' explosions and it will not be easy to find those responsible.
"We are carrying out a number of concrete investigative precautions ... Our hope is to be able to confirm who has committed this crime, but it should be noted that it likely will be difficult given the circumstances," Ljungqvist said, as quoted by the Swedish Prosecution Authority, adding that the crime is "difficult to investigate," as it "took place 80 meters [262 feet] under the water."
The Nord Stream and Nord Stream 2 gas pipelines, built to deliver gas under the Baltic Sea from Russia to Germany, were hit by explosions last September. Nord Stream's operator, Nord Stream AG, said that the damage was unprecedented and it was impossible to estimate the time repairs might take.
Russia considers the explosions of the two pipelines an act of international terrorism. There are no official results of the investigation yet.
US journalist and Pulitzer Prize winner Seymour Hersh alleged that the explosions were organized by the United States with the support of Norway. Following Hersh's report, Moscow reiterated its calls for an impartial and thorough investigation. The US has denied its involvement in the incident.