Germany treads a careful line over reports that a pro-Ukrainian group sabotaged the Nord Stream pipeline.

German officials were treading a careful line on Wednesday in response to reports that a pro-Ukrainian group could be behind last year’s attack on the Nord Stream natural gas pipeline that links Russia and Germany, saying that they were waiting for the results of further investigations.

The New York Times reported that new intelligence reviewed by U.S. officials suggested that the attack, in September of last year, was carried out by pro-Ukrainian perpetrators, and similar reports followed in the German news media. U.S. officials told The Times that they had no evidence that the attack was done at the direction of the Ukrainian government, and Kyiv has denied any responsibility.

Boris Pistorius, Germany’s defense minister, said on Wednesday, without elaborating, that the attack could also have been a “false-flag action” to make it appear that it had been carried out by pro-Ukrainian groups.

“The likelihood of either is equally high,” he told a German public broadcaster, saying that he would wait on further developments before commenting on what he called “hypothetical research.”

Germany has delivered more weapons than any other European Union country to Ukraine, and although many Germans back the government’s support, there are fears that sending weapons into a war zone will lengthen the conflict and risk setting off a wider war. Any hint that Ukraine’s government was behind the pipeline attack could make it harder for Chancellor Olaf Scholz to continue delivering heavy weapons and providing training for Ukrainian soldiers.

Suspicion over the attack — which affected three of the four pipelines that make up Nord Stream 1 and 2 — immediately fell on Moscow, but speculation about who was to blame has included Kyiv, London and Washington.

The attack compounded fears in Germany of a gas shortage during the winter. Fifty-five percent of all gas used in Germany before the war had come from Russia, and although in the months after the invasion Germany had weaned itself off much of the natural gas it was receiving directly from Russia, the explosions made it clear that there was no going back.

Ukraine has rejected the reports linking a pro-Ukrainian group to the pipeline attack. Mykhailo Podolyak, an adviser to President Volodymyr Zelensky, posted on Twitter: “Although I enjoy collecting amusing conspiracy theories about Ukrainian government, I have to say: Ukraine has nothing to do with the Baltic Sea mishap and has no information about ‘pro-Ukrainian sabotage groups.’”

Russia welcomed the reports that Moscow was not involved, though officials continued to point a finger at Western countries. Andrey Ledenev, an official with the Russian Embassy in Washington, suggested that the report was an attempt to “shift the blame from the statesmen who ordered and coordinated the attacks in the Baltic Sea to some abstract individuals,” according to the Russian news agency Tass.