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WASHINGTON — New intelligence reviewed by U.S. officials suggests that a pro-Ukrainian group carried out the attack on the Nord Stream pipelines last year, a step toward determining responsibility for an act of sabotage that has confounded investigators on both sides of the Atlantic for months.

U.S. officials said that they had no evidence President Volodymyr Zelensky of Ukraine or his top lieutenants were involved in the operation, or that the perpetrators were acting at the direction of any Ukrainian government officials.

The brazen attack on the natural gas pipelines, which link Russia to Western Europe, fueled public speculation about who was to blame, from Moscow to Kyiv and London to Washington, and it has remained one of the most consequential unsolved mysteries of Russia’s year-old war in Ukraine.

Ukraine and its allies have been seen by some officials as having the most logical potential motive to attack the pipelines. They have opposed the project for years, calling it a national security threat because it would allow Russia to sell gas more easily to Europe. Ukrainian government and military intelligence officials say they had no role in the attack and do not know who carried it out.

U.S. officials said there was much they did not know about the perpetrators and their affiliations. The review of newly collected intelligence suggests they were opponents of President Vladimir V. Putin of Russia, but does not specify the members of the group, or who directed or paid for the operation. U.S. officials declined to disclose the nature of the intelligence, how it was obtained or any details of the strength of the evidence it contains. They have said that there are no firm conclusions about it, leaving open the possibility that the operation might have been conducted off the books by a proxy force with connections to the Ukrainian government or its security services.

Some initial U.S. and European speculation centered on possible Russian culpability, especially given its prowess in undersea operations, though it is unclear what motivation the Kremlin would have in sabotaging the pipelines given that they have been an important source of revenue and a means for Moscow to exert influence over Europe. One estimate put the cost of repairing the pipelines starting at about $500 million. U.S. officials say they have not found any evidence of involvement by the Russian government in the attack.

The All Access sale is on. Save now. £2.40 a month for your first year. Limited time offer. Officials who have reviewed the intelligence said they believed the saboteurs were most likely Ukrainian or Russian nationals, or some combination of the two. U.S. officials said no American or British nationals were involved.

The pipelines were ripped apart by deep sea explosions in September, in what U.S. officials described at the time as an act of sabotage. European officials have publicly said they believe the operation that targeted Nord Stream was probably state sponsored, possibly because of the sophistication with which the perpetrators planted and detonated the explosives on the floor of the Baltic Sea without being detected. U.S. officials have not stated publicly that they believe the operation was sponsored by a state.

The explosives were most likely planted with the help of experienced divers who did not appear to be working for military or intelligence services, U.S. officials who have reviewed the new intelligence said. But it is possible that the perpetrators received specialized government training in the past.

Officials said there were still enormous gaps in what U.S. spy agencies and their European partners knew about what transpired. But officials said it might constitute the first significant lead to emerge from several closely guarded investigations, the conclusions of which could have profound implications for the coalition supporting Ukraine.

Any suggestion of Ukrainian involvement, whether direct or indirect, could upset the delicate relationship between Ukraine and Germany, souring support among a German public that has swallowed high energy prices in the name of solidarity.

Nord Stream 1 and Nord Stream 2, as the two pipelines are known, stretch 760 miles from the northwest coast of Russia to Lubmin in northeast Germany. The first cost more than $12 billion to build and was completed in 2011.

Nord Stream 2 cost slightly less than the first pipeline and was completed in 2021, over objections from officials in the United States, Britain, Poland and Ukraine, among others, who warned that it would increase German reliance on Russian gas. During a future diplomatic crisis between the West and Russia, these officials argued, Moscow could blackmail Berlin by threatening to curtail gas supplies, on which the Germans had depended heavily, especially during the winter months. (Germany has weaned itself off reliance on Russian gas over the past year.)

Early last year, President Biden, after meeting with Chancellor Olaf Scholz of Germany at the White House, said Mr. Putin’s decision about whether to attack Ukraine would determine the fate of Nord Stream 2. “If Russia invades, that means tanks and troops crossing the border of Ukraine again, then there will be no longer a Nord Stream 2,” Mr. Biden said. “We will bring an end to it.”

When asked exactly how that would be accomplished, Mr. Biden cryptically said, “I promise you we’ll be able to do it.”

A couple weeks later, Mr. Scholz announced that his government would block the Nord Stream 2 pipeline from becoming operational. Two days after that, Russia launched the much-anticipated invasion.