The United States and Britain urged Russia on Friday to bring to justice all those responsible for the slaying of opposition politician Boris Nemtsov. A Russian court on Thursday convicted five men of murdering Nemtsov, but allies of the politician have said the investigation was a cover-up and that the people who had ordered his killing remained at large. “We call once more on the Russian government to ensure that all involved in the killing of Boris Nemtsov, including anyone involved in organizing or ordering the crime, are brought to justice,” the U.S. State Department said in a statement, calling Nemtsov a “champion of democracy and human rights.” Britain said it wanted Russia to further investigate Nemtsov’s death. “The Foreign and Commonwealth Office supports Boris Nemtsov’s family in their call for a fuller investigation into who ordered his murder,” a spokesman for Britain’s Foreign Ministry said. “Responsibility for his murder …