Initial investigations into the Nord Stream pipeline leaks have reinforced the theory that "serious sabotage" was responsible.
Sweden’s domestic security agency on Thursday said its preliminary inquiry into the leaks from two Russian gas pipelines in the Baltic Sea “has strengthened the suspicions of serious sabotage” as the cause.
The Swedish Security Service said investigators had confirmed that “detonations" caused extensive damage to the Nord Stream 1 and 2 pipelines last week.
Authorities had said when the leaks first surfaced near Sweden and Denmark that explosions were recorded in the area.
In a separate statement, Swedish prosecutor Mats Ljungqvist said: “Seizures have been made at the crime scene and these will now be investigated.”
Mr Ljungqvist, who led the preliminary investigation, did not identify the seized evidence but said that now the initial probe was complete, a blockade around the pipelines off Sweden would be lifted.
The Danish and Swedish governments previously said they suspected that several hundred pounds of explosives were involved in carrying out a deliberate act of sabotage.
The leaks from Nord Stream 1 and 2 discharged large amounts of methane into the air.
Mr Ljungqvist called it “a serious incident” and said “the case is very sensitive.”
Last week, undersea explosions ruptured Nord Stream 1 and its sister pipeline Nord Stream 2 at two locations off Sweden and two off Denmark.
The pipelines were built to carry Russian natural gas to Germany.
Russian President Vladimir Putin accused the West of attacking the pipelines, which the United States and its allies vehemently denied.
A Kremlin representative on Thursday said he did not think the investigation could be objective without Russia's participation — it was not invited to take part.
Danish authorities said the two leaks they were monitoring in international waters had stopped at the weekend. One of the leaks off Sweden also appeared to have ended.
Police in Copenhagen were leading Denmark's inquiry in co-operation with energy authorities, the National Police and the Danish Police Intelligence Service.
As a result of the incident, Britain has stepped up its security surrounding energy infrastructure.