FBI warns Trump and Clinton staffers to beware of foreign spies in U.S.

An FBI official described the sessions as voluntary “awareness briefings” provided to “low-level staffers” of the two major-party campaigns every presidential election cycle.

Each briefing Wednesday – a morning session for the Trump team and a later one for the Clinton team – lasted about an hour, ABC News was told.

One campaign source said FBI agents laid out a “standard best practices briefing,” emphasizing it was “not precipitated by any particular [or known] threat.”

Nevertheless, the FBI official acknowledged the unique threat environment facing the United States, particularly espionage efforts from Russia.