Sweden’s domestic security agency has said its preliminary investigation of 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 the probe confirmed that “detonations” caused extensive damage to the Nord Stream 1 and Nord Stream 2 pipelines last week.

Authorities had said when the leaks off Sweden and Denmark first surfaced that explosions were recorded in the area.

The agency did not give details about its investigation, but in a separate statement Swedish prosecutor Mats Ljungqvist said: “Seizures have been made at the crime scene and these will now be investigated.”

Europe Pipelines
Pipe systems and shut-off devices at the gas receiving station of the Nord Stream 2 Baltic Sea pipeline in Lubmin, Germany (Stefan Sauer/dpa/AP)

Mr Ljungqvist, who led the preliminary investigation, did not identify the seized evidence, but he said now that the initial inquiry has been completed, a blockade around the pipelines off Sweden will be lifted.

The governments of Denmark and Sweden 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”. He declined to elaborate, saying there is “pre-trial confidentiality”.

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.

Danish authorities said the two leaks they were monitoring in international waters stopped over the weekend. One of the leaks off Sweden also appeared to have ended.

Copenhagen police were leading Denmark’s inquiry in co-operation with energy authorities, the National Police and the Danish Police Intelligence Service.