U.S. officials say they are investigating a phone call between President Trump’s national security adviser, Michael Flynn, and Russia’s ambassador to the United States. The call was on December 29—the same day President Obama imposed sanctions against Russia and expelled diplomats, in retaliation for the alleged Russian hacking of the U.S. election. U.S. officials are also investigating a handful of Donald Trump’s close associates, including former campaign manager Paul Manafort, over possible ties to Russia.