BRUSSELS — The mysterious blasts in September that made the largest-capacity natural gas pipelines from Russia to Europe inoperable were caused by “gross sabotage,” Swedish authorities confirmed Friday, noting that traces of explosives have been found as part of the ongoing investigation.“The advanced analysis work is still in progress — the aim is to draw more definitive conclusions about the Nord Stream incidents,” the Security Service statement said.But European leaders suggested the explosions were intended as a threat, sending a message that their critical infrastructure could be vulnerable if they continued their support of Ukraine.Russian energy giant Gazprom stopped the gas flow through Nord Stream 1 earlier this year, citing technical problems, while European leaders accused Moscow of “blackmail.” The newer Nord Stream 2 pipeline was not yet approved for operation; Germany froze the project in the lead-up to the war in Ukraine.Russia’s Gamble: The Post examined the road to war in Ukraine, and Western efforts to unite to thwart the Kremlin’s plans, through extensive interviews with more than three dozen senior U.S., Ukrainian, European and NATO officials."