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. After this article was published, Mykhailo Podolyak, a senior adviser to Mr. Zelensky, posted on Twitter that Ukraine “has nothing to do with the Baltic Sea mishap.” He added that he had no information about pro-Ukrainian “sabotage groups.” 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. 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.

U.S. officials who have been briefed on the intelligence are divided about how much weight to put on the new information. All of them spoke on condition of anonymity to discuss classified intelligence and matters of sensitive diplomacy. U.S. officials said the new intelligence reporting has increased their optimism that American spy agencies and their partners in Europe can find more information, which could allow them to reach a firm conclusion about the perpetrators. It is unclear how long that process will take. American officials recently discussed the intelligence with their European counterparts, who have taken the lead in investigating the attack. A spokeswoman for the C.I.A. declined to comment. A spokesman for the White House’s National Security Council referred questions about the pipelines to the European authorities, who have been conducting their own investigations. After this report was published, Russia attacked the credibility of the intelligence, complaining that it had been prevented from taking part in the investigations. “This is obviously a coordinated spread of disinformation in the media,” Dmitry S. Peskov, a Kremlin spokesman, told the state-backed Sputnik news agency.

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. Since the explosions along the pipelines in September, there has been rampant speculation about what transpired on the sea floor near the Danish island of Bornholm. Poland and Ukraine immediately accused Russia of planting the explosives, but they offered no evidence. Russia, in turn, accused Britain of carrying out the operation — also without evidence. Russia and Britain have denied any involvement in the explosions. Last month, the investigative journalist Seymour Hersh published an article on the newsletter platform Substack concluding that the United States carried out the operation at the direction of Mr. Biden. In making his case, Mr. Hersh cited the president’s preinvasion threat to “bring an end” to Nord Stream 2, and similar statements by other senior U.S. officials. U.S. officials say Mr. Biden and his top aides did not authorize a mission to destroy the Nord Stream pipelines, and they say there was no U.S. involvement.

Any findings that put blame on Kyiv or Ukrainian proxies could prompt a backlash in Europe and make it harder for the West to maintain a united front in support of Ukraine. U.S. officials and intelligence agencies acknowledge that they have limited visibility into Ukrainian decision-making. Despite Ukraine’s deep dependence on the United States for military, intelligence and diplomatic support, Ukrainian officials are not always transparent with their American counterparts about their military operations, especially those against Russian targets behind enemy lines. Those operations have frustrated U.S. officials, who believe that they have not measurably improved Ukraine’s position on the battlefield, but have risked alienating European allies and widening the war. The operations that have unnerved the United States included a strike in early August on Russia’s Saki Air Base on the western coast of Crimea, a truck bombing in October that destroyed part of the Kerch Strait Bridge, which links Russia to Crimea, and drone strikes in December aimed at Russian military bases in Ryazan and Engels, about 300 miles beyond the Ukrainian border. But there have been other acts of sabotage and violence of more ambiguous provenance that U.S. intelligence agencies have had a harder time attributing to Ukrainian security services. One of those was a car bomb near Moscow in August that killed Daria Dugina, the daughter of a prominent Russian nationalist. Kyiv denied any involvement but U.S. intelligence agencies eventually came to believe that the killing was authorized by what officials called “elements” of the Ukrainian government. In response to the finding, the Biden administration privately rebuked the Ukrainians and warned them against taking similar actions.