A suspected underwater sabotage attack off Europe's shores in September left politicians, security officials and scientists looking for answers to the mystery of what caused two majority Russian-controlled pipelines to leak gas into the Baltic Sea.
Underwater blasts which rattled the Baltic seabed shortly before the pipeline leaks led to the suspicion of sabotage.
The tremors were picked up by scientists in Germany, Denmark and Sweden, who described two separate spikes on their seismographs.
Denmark’s geological survey indicated they were man-made and matched the locations where leaks were reported on the Nord Stream pipelines.
“This is not our activity,” Defence Minister Oleksiy Reznikov told reporters in Stockholm on Tuesday, before a meeting with EU defence ministers.
Ukrainian presidential adviser Mykhailo Podolyak also tweeted that “Ukraine has nothing to do with the Baltic Sea mishap and has no information about 'pro-Ukraine sabotage groups'.”
A separate German investigation has not ruled out the possibility of a “false flag” attack by Russia to blame Ukraine.
Authorities in Sweden and Denmark have also opened investigations into the incident.
What do we know about what actually happened?
The pipelines were ruptured by subsea explosives on September 26 last year, seven months after Russian forces invaded Ukraine.
Several leaks were reported in both Nord Stream 1 and Nord Stream 2, in Danish and Swedish waters. The gas bubbling at the surface could be seen from space.
According to a German investigation, a group of five men and one woman carried out the attack and used professionally- falsified passports to cover their tracks.
The boat used by the group has been identified and it is believed to have set sail from the north German port of Rostock on September 6 last year, landing the following day on the Danish island of Christianso.
Traces of explosives were later found by investigators as the team returned the boat with having cleaned up.
What was the impact?
The attack benefited Ukraine by severely damaging Russia's means of reaping millions by selling natural gas to Western Europe.
But it added to the pressure of high energy prices on key Ukrainian allies in Europe, particularly Germany.
Three weeks after the leaks were discovered, gas prices, which had already soared following Russia's invasion of Ukraine, jumped a further 8 per cent.
The wholesale cost of energy has since fallen, although it remains higher for consumers than before Russia's invasion of Ukraine.
The blast also had a significant environmental impact.
The pipelines were estimated to have contained 300,000 tons of methane and it is believed to be one of the largest single releases of methane in history, equivalent to one and a half days of global emissions.
Who did it?
A blog post by an award-winning US political writer in February claimed the US was behind the Nord Stream gas pipeline explosions, an accusation the White House described as “utterly false and complete fiction”.
And now, a New York Times report claims US officials have seen new intelligence that indicates a “pro-Ukrainian group” was responsible.
It suggested the perpetrators behind the sabotage were “opponents of President Vladimir Putin of Russia”, and probably Ukrainian or Russian citizens.
The report said US officials had no evidence implicating Ukrainian President Volodymyr Zelenskyy, and that it may have been carried out “off the books” by a proxy force with connections to the Ukrainian government or its security services.
Ukraine has denied any involvement in the operation.
