U.S. officials are looking into whether a pro-Ukrainian group conducted the attack on the Nord Stream gas pipelines last year, according to a report from The New York Times.

The New York Times reported Monday that U.S. officials are reviewing intelligence suggesting that a group supportive of Ukraine attacked the natural gas pipelines; however, there are no apparent ties between that group and the Ukrainian government.

The Nord Stream pipelines connect Russia to Germany and Western Europe for the delivery of natural gas. They have become a major flashpoint in the war between Russia and Ukraine, along with its impact on energy prices in Europe.

Explosions in the Baltic Sea last September damaged the pipelines, leading to finger-pointing across the globe. Several top Western leaders suggested that Russia could be behind the attack in the wake of the explosions.

The Ukrainian government has denied any involvement in the attack, but a number of U.S. officials have thought for some time that the country could be the most likely perpetrator, according to the Times. Ukraine opposed the creation of the pipeline.

The new intelligence, according to the Times, suggests that the attack was carried out by opponents of Russian President Vladimir Putin, but officials said it did not specify the members or the funders of the group. Officials also declined to disclose how the intelligence was gathered or the strength of the evidence. 

Europe’s decades-long dependence on Russian oil and natural gas has come back to bite key Ukrainian allies. The combination of sanctions on Russian energy and shocks to global supply of oil and natural gas caused a steep increase in energy costs in Europe, along with a lesser bump in the U.S.