European security officials on Monday and Tuesday observed Russian Navy ships in the vicinity of leaks in the Nord Stream pipelines likely caused by underwater explosions, according to Western intelligence officials and one other source.
It's unclear whether the ships had anything to do with those explosions, these sources and others said -- but it's one of the many factors that investigators will be looking into.
Russian submarines were also observed not far from those areas last week, one of the intelligence officials said.
Three US officials said that the United States has no thorough explanation yet for what happened, days after the explosions that caused three separate and simultaneous leaks in the two pipelines on Monday.
Russian ships routinely operate in the area, according to one Danish military official, who emphasized that the presence of the ships doesn't necessarily indicate that Russia caused the damage.
"We see them every week," this person said. "Russian activities in the Baltic Sea have increased in recent years. They're quite often testing our awareness – both at sea and in the air."
But the sightings still cast further suspicion on Russia, which has drawn the most attention from both European and US officials as the only actor in the region believed to have both the capability and motivation to deliberately damage the pipelines.
US officials declined to comment on the intelligence about the ships on Wednesday.
The prime ministers of both Denmark and Sweden said on Tuesday that the leaks were likely the result of deliberate actions, not accidents, and Sweden's security service said in a statement Wednesday that it cannot be ruled out "that a foreign power is behind it."
US National Security Adviser Jake Sullivan on Tuesday evening also called the leaks "apparent sabotage" in a tweet.
But senior Western officials have so far stopped short of attributing the attack to Russia or any other nation.
The Kremlin has publicly denied striking the pipelines. A spokesman called the allegation "predictably stupid and absurd."
CNN has reached out to the Russian Ministry of Defense for comment on the presence of the ships.
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CNN's Oren Liebermann and Alex Marquardt contributed reporting.