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COPENHAGEN, Denmark (AP) — Sweden’s prime minister says that her country cannot share with Russia details from its probe into last month’s underwater explosions that ruptured two key gas pipelines in the Baltic Sea, citing confidentiality surrounding the investigation.

“In Sweden there is secrecy around preliminary investigation and that also applies in this case,” Magdalena Andersson said of the blast and ruptures that happened in international waters off Sweden’s Baltic coastline but within the country’s exclusive economic zone.

The explosions ruptured the Nord Stream 1 pipeline, which until Russia cut off supplies at the end of August was its main gas supply route to Germany. They also damaged the Nord Stream 2 pipeline, which never entered service as Germany suspended its certification process shortly before Russia invaded Ukraine in February. The damaged pipelines discharged huge amounts of methane, a potent greenhouse gas, into the air.

Russia formally asked Sweden’s government to be part of the Swedish investigation in a letter dated Oct. 6.

“We’re still working on how we exactly formulate the answer,” Andersson said Monday at a naval base in southern Sweden.

In its preliminary investigation, Sweden’s domestic security agency said last week that its probe “has strengthened the suspicions of serious sabotage” as the cause of the blasts. Sweden’s prosecutor in charge of the investigation said evidence at the site has been seized.

The Swedish Security Service said the probe confirmed that “detonations” caused extensive damage to the pipelines. Authorities had said when the four leaks off Sweden and Denmark first surfaced 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.” Ljungqvist, who led the preliminary investigation, did not identify the seized evidence.

In Denmark, authorities remained tight-lipped about its investigation. Denmark broadcaster TV2 reported from the site that ships with the Danish and German navy ships were in the area.

German federal prosecutors, who investigate national security cases, also have opened an investigation against persons unknown on suspicion of deliberately causing an explosion and anti-constitutional sabotage.

The German investigation comes on top of the Danish and Swedish probes but are carried out with the European Union framework.

German federal prosecutors said the reason for them getting involved as well is that an attack on energy supplies could affect Germany’s external and domestic security. On Sunday, authorities said that two German boats had set off for the area where the leaks occurred to look into what happened.

Russian President Vladimir Putin has accused the West of attacking the pipelines, which the United States and its allies vehemently denied.