Explosives Found in Russian Jet Wreckage MOSCOW - Traces of explosives have been found in the wreckage of one of two airliners that crashed nearly simultaneously earlier this week, the Federal Security Service said Friday, a day after a top official acknowledged that terrorism was the most likely cause of the crashes.    A duty officer at the agency, the main successor to the Soviet-era KGB, confirmed reports on Russian news agencies that cited agency spokesman Sergei Ignatchenko as saying that "preliminary analysis indicates it was hexogen."   The announcement came several hours after a Web site known for militant Muslim published a claim of responsibility for the twin crashes, connecting the action to Russia's fight against separatists in Chechnya...