After explosions ruptured the Nord Stream pipelines and set the Baltic Sea boiling with leaking methane gas, US and European officials were quick to blame Russia.
Four months on, investigators are unable to prove Moscow was behind the attack, but officials say the explosions illustrate the threat malign actors — especially Russia — pose to vital undersea infrastructure.
After the explosions in late September, skeptics argued that Russia had little to gain in severing pipelines that gave it leverage over European energy supplies, but numerous officials said Russia was the likely culprit, citing Moscow's reliance on unconventional warfare and rising tensions with its European neighbors.
As of late December, however, there was "no evidence at this point that Russia was behind the sabotage," a European official told The Washington Post, echoing the views of nearly two dozen diplomatic and intelligence officials from nine countries.
Investigators have confirmed the blasts were deliberate, but the admiral in charge of the Office of Naval Intelligence acknowledged this month that there are "still some unknowns," including the perpetrator.
"Obviously, we have a number of investigations underway with different countries taking a look at it," Rear Adm. Michael Studeman said at an Intelligence and National Security Alliance event on January 11.
"The sabotage is confirmed based on what we know so far, but we haven't ruled out any guilty party at this stage of the game," Studeman added. "So we're going to have to wait and see what the evidence and where this investigation or series of investigations go, so stand by right now, but we don't know enough to make any conclusions."
Despite the uncertainty, the attack has only added to concern about threats to undersea infrastructure, particularly cables and pipelines, that connects continents and powers economies.
"There is a vulnerability around anything that sits upon the seabed, whether that's gas pipelines, whether that's data cables," Adm. Sir Ben Key, first sea lord and chief of the British naval staff, said aboard British aircraft carrier HMS Queen Elizabeth just days after the blast.
That vulnerability is a longstanding British concern. In 2015, the chief of the UK defense staff described threats to cables and pipelines as "a new risk to our way of life." In a 2017 report, Rishi Sunak, then a member of British parliament, described undersea internet cables as "indispensable yet insecure" — a theme Sunak revisited in November in his first major foreign-policy speech as prime minister.
Russia is seen as uniquely capable of interfering with that infrastructure. It has a number of submarines with special-mission capabilities, including the ability to deploy smaller submersibles to tap cables or meddle with pipelines.
US military officials say they have seen worrying increases in Russian activity around that infrastructure, and Washington has imposed sanctions to counter Russian investment in such activity.