WASHINGTON — American and European officials believe that Russian military intelligence officers directed associates of a white supremacist militant group based in Russia to carry out a recent letter bomb campaign in Spain whose most prominent targets were the prime minister, the defense minister and foreign diplomats, according to U.S. officials. Spanish and foreign investigators have been looking into who sent six letter bombs in late November and early December to sites mostly in Madrid, including the official residence of Prime Minister Pedro Sánchez, which also serves as his office; the American and Ukrainian Embassies; and the Defense Ministry. No...