On Friday, Public Prosecutor Mats Ljungqvist said that the Nord Stream gas pipelines, which were built to transport gas from Russia to Germany, were deliberately sabotaged.
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"During the crime scene investigations that were carried out on-site in the Baltic Sea, extensive seizures were made and the area has been carefully documented," he announced.
"Analyses that have now been carried out show residues of explosives on several of the foreign objects that were found. The advanced analysis work continues to be able to draw safer conclusions about the incident."
Four leaks were discovered in the Swedish and Danish exclusive economic zones of the Baltic Sea in September, and Sweden soon opened an investigation into gross sabotage.
An analysis of the seismic activity in the area showed seismic patterns corresponding to explosions on the seabed and evidence was gathered with the assistance of the Swedish Navy.
The Swedish Prosecution Authority said that the "complex and extensive" investigation will show whether anyone can be served with suspicion of a crime.
"The cooperation with authorities in Sweden and other countries works excellently. For the continued work with the preliminary investigation and for the various ongoing collaborations, it is important that we can work in peace and quiet," Ljungqvist said.
Germany and Denmark have also launched investigations into what caused the leaks in the twin pipelines. Russia has demanded that Sweden share the findings of the investigation, but the Swedish authorities have refused to do so, citing pre-trial confidentiality.