MOSCOW/WASHINGTON (Reuters) -Russian investigators said on Thursday they had found proof that gunmen who killed more than 140 people at a concert last week were linked to "Ukrainian nationalists", an assertion immediately dismissed by the United States as baseless propaganda. Russia has said from the outset that it believes Ukraine was linked to the attack, even though Kyiv has denied it and the militant group Islamic State has claimed responsibility. In a statement, Russia's Investigative Committee said for the first time that it had uncovered proof of a Ukrainian connection.