The Spanish police arrested a 74-year-old man on suspicion of sending letter bombs to the prime minister’s office, the American and Ukrainian embassies and a weapons manufacturer, among other sites, Spain’s Interior Ministry said on Wednesday. American officials had said previously that they suspected Russian military intelligence officers had directed associates of a white supremacist militant group to carry out the campaign. But the Spanish Interior Ministry, in its statement about the arrest, did not mention any possible link between the detained man and either far-right groups or Russia. According to the ministry, the Spanish authorities believe that the arrested...