Pro-Ukrainian group blew up controversial Russian gas pipelines, but it's unclear who sent them, reports say

• A pro-Ukrainian group was behind the Nord Stream pipeline sabotage, per new reports.
• But it's still unclear who sent the group to carry out the underwater bombings.

A pro-Ukrainian group was responsible for the undersea sabotage of the Nord Steam pipelines last September, according to new intelligence reviewed by US officials that was first reported on by the New York Times.

The German newspaper Die Zeit also reported that investigators believe they've located the yacht used in the operation. Investigators found traces of explosives on a table in the vessel, and the yacht was rented from a Poland-based company that is seemingly owned by two Ukrainians, the report said. Echoing the Times report, Die Zeit also said that intelligence indicated a pro-Ukrainian group was behind the sabotage.

But both reports emphasized that it's unclear who ordered the bombings, and neither implicated Ukrainian President Volodymyr Zelenskyy.

Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskyy, said his government had no involvement in the attacks. "Although I enjoy collecting amusing conspiracy theories about the Ukrainian government, I have to say: Ukraine has nothing to do with the Baltic Sea mishap and has no information about 'pro-Ukraine sabotage groups,'" he tweeted.

Six people took part in the operation, according to Die Zeit, including the vessel's captain, two divers, two diving assistants, and a doctor. Officials who've seen the new intelligence believe the perpetrators were most likely Ukrainian or Russian nationals, or a combination of both, the Times said.

NATO and the US have said the pipeline blasts in September were acts of sabotage but did not assign blame. Ukraine accused Russia of being behind the attacks while the Kremlin has blamed the West.

The White House on Tuesday responded to the new reports with caution.

"We need to let these investigations conclude, and only then should we be looking at what follow-on actions might or may not be appropriate," White House spokesperson John Kirby said, per Reuters. Meanwhile, the German government on Tuesday said it had not yet concluded its investigation into the sabotage.

In discussions with the Times, US officials refused to detail the nature of the intelligence, how it was obtained, or the level of confidence behind it, only offering that the conclusions officials arrived at after reviewing the intelligence are not firm.

The Nord Steam 1 and Nord Stream 2 pipelines, which transported natural gas from Russia to Germany, were controversial even before Russian President Vladimir Putin ordered the unprovoked invasion of Ukraine last year. Critics of the pipelines, which included Ukraine, felt they gave Russia far too much influence over Europe's energy supply. The Russian state-owned company Gazprom built the pipelines.

Nord Stream 1 was shut down indefinitely last September, and Nord Steam 2 was never operational. Facing immense pressure to end its reliance on energy from Russia, Germany froze the Nord Stream 2 project two days before Russian forces invaded Ukraine.