The Defense Department has filed a formal protest against the Russian government, after one of its air force’s Sukhoi Su-27 fighters buzzed an Air Force RC-135U reconnaissance plane on April 7. The RC-135 was conducting a mission in international territory near the western Russian city of Kaliningrad, when the Sukhoi aircraft came within less than 50 feet of it. Russia Today, a state-sponsored news service, reported that Russian officials claimed the U.S. plane was flying with its transponder turned off. U.S. officials countered on Twitter that the transponder was indeed on.