A preliminary investigation into gas leaks from two underwater pipelines connecting Russia to Germany found “powerful explosions” caused the damage, Copenhagen Police said Tuesday.

The findings appeared to be similar to a crime scene investigation carried out by Sweden’s national security service earlier this month, which reinforced suspicions of “gross sabotage.”

A flurry of detonations on the Nord Stream 1 and 2 pipelines on Sept. 26 sent gas spewing to the surface of the Baltic Sea. The explosions triggered four gas leaks at four locations — two in Denmark’s exclusive economic zone and two in Sweden’s exclusive economic zone.

Danish police said a joint group, including The Norwegian Police Intelligence Service, would be set up to handle further investigations of the incidents.

“It is still too early to say anything about the framework under which the international cooperation with e.g. Sweden and Germany will run, as it depends on several actors, including which authorities handle the case in the various countries,” the statement said Tuesday.

Danish police said it was not possible to say when the investigation was likely to be completed.

Many in Europe suspect the Nord Stream gas leaks were the result of an attack, particularly as it occurred during a bitter energy standoff between the European Union and Russia.

The Kremlin has repeatedly dismissed claims it destroyed the pipelines, calling such allegations “stupid” and “absurd,” and claiming that it is the U.S. that had the most to gain from the gas leaks.

The White House has denied any involvement in the suspected attack.