Seymour Hersh broke the story on this. He's famous for breaking the My Lai massacre. The article he wrote is behind a pay wall, but this is a very lengthy interview with from democracy now https://www.democracynow.org/2023/2/15/nord_stream_sy_hersh

Here's a Washington post article including statements from the pentagon claiming that they've been aware for over a year that the Ukrainian military had plans to destroy Nord Stream 2 and then subsequently did: https://www.washingtonpost.com/national-security/2023/06/06/nord-stream-pipeline-explosion-ukraine-russia/

Three months before saboteurs bombed the Nord Stream natural gas pipeline, the Biden administration learned from a close ally that the Ukrainian military had planned a covert attack on the undersea network, using a small team of divers who reported directly to the commander in chief of the Ukrainian armed forces.

Details about the plan, which have not been previously reported, were collected by a European intelligence service and shared with the CIA in June 2022. They provide some of the most specific evidence to date linking the government of Ukraine to the eventual attack in the Baltic Sea, which U.S. and Western officials have called a brazen and dangerous act of sabotage on Europe’s energy infrastructure.

...

The highly specific details, which include numbers of operatives and methods of attack, show that for nearly a year, Western allies have had a basis to suspect Kyiv in the sabotage. That assessment has only strengthened in recent months as German law enforcement investigators uncovered evidence about the bombing that bears striking similarities to what the European service said Ukraine was planning.

...

Ukrainian officials, who have previously denied the country was involved in the Nord Stream attack, did not respond to requests for comment.

The White House declined to comment on a detailed set of questions about the European report and the alleged Ukrainian military plot, including whether U.S. officials tried to stop the mission from proceeding.

The CIA also declined to comment.

...

The European intelligence made clear that the would-be attackers were not rogue operatives. All those involved reported directly to Gen. Valery Zaluzhny, Ukraine’s highest-ranking military officer, who was put in charge so that the nation’s president, Volodymyr Zelensky, wouldn’t know about the operation, the intelligence report said.

imo this is just bullshit and the pentagon is just trying to throw the Ukrainian government under the bus to save face that the US committed an act of war against a fellow NATO member - the Hersh theory seems a lot more likely since it's not just uncritically repeating the current public line of the Pentagon. Still the article illustrates how it was very definitely either the US or a close US ally who did the bombing

It may have been Ukraine with America's assistance, fair