According to current and former US officials cited by Reuters, it’s easy to access commercial encryption technology and the increased presence of Russian agents in the US has made it hard for the FBI to keep up with them.
"It’s more complex now. The complexity comes in the techniques that can be used," one of the officials said.
Earlier the United States and a number of other countries expelled more than 150 Russian diplomats in response to the alleged nerve agent attack against Skripal and his daughter Yulia in Salisbury, England. London described the move as the "biggest expulsion of Russian spies since the Cold War."
Former FBI espionage chief Michael Rochford warned, however, that the mass expulsion of "spies" posing as diplomats might pose a problem for US security services as it won’t be immediately clear who might be a spy among the replacements Moscow will send.