Ukraine’s interim Interior Minister Arsen Avakov has authorized the detention and questioning of the leaders of radical Right Sector party after receiving threats of retaliation for the killing of one of its notorious activists, a ministry’s source said Tuesday. The source said on condition of anonymity that Avakov’s order includes a police sweep of Kiev's Independence Square, known as Maidan, which is still occupied by scores of ultranationalists. “Investigators from the ministry are already working at some of the tents on Maidan,” the source said, adding that a crackdown on Maidan could start later tonight. Ukrainian special forces have gunned...