The WaPo story cites "officials" without identifying them as US officials. Maybe just an oversight on their part, but probably what Rosenstein is referring to. Either way, WaPo needs to clarify which country these "officials" represent.
"The special counsel overseeing the investigation into Russia’s role in the 2016 election is interviewing senior intelligence officials as part of a widening probe that now includes an examination of whether President Trump attempted to obstruct justice, officials said."
https://www.washingtonpost.com/world/national-security/special-counsel-is-investigating-trump-for-possible-obstruction-of-justice/2017/06/14/9ce02506-5131-11e7-b064-828ba60fbb98_story.html?utm_term=.9aa6a3742c4c
