European security officials on Monday and Tuesday observed Russian Navy support ships in the vicinity of leaks in the Nord Stream pipelines likely caused by underwater explosions, according two Western intelligence officials and one other source familiar with the matter, writes CNN.

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 US has no thorough explanation yet for what happened, days after the explosions appeared to cause 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. 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”, - this person said.

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.