Washington( CNN) Diplomatic relations between the U.S. and Russia began to unravel after Moscow told Saturday it had expelled two American envoys and the U.S. exposed it had tossed two Russians in response to an attack on one of its personnel.

“On June 17, we expelled two Russian officials from the United States to respond to this attack, ” State Department spokesman John Kirby told CNN, referring to an American diplomat who the spokesman told “was attacked by a Russian policeman” while trying to enter the U.S. embassy last month in Moscow.