General view of the U.S. Embassy in Havana after the U.S. government pulled more than half of its diplomatic personnel out of Cuba in September 2017. (Photo: Ernesto Mastrascusa/Getty Images)
Primer:
Intelligence agencies investigating mysterious “attacks” that led to brain injuries in U.S. personnel in Cuba and China consider Russia to be the main suspect, three U.S. officials and two others briefed on the investigation tell NBC News.