The officials do not believe the sources have been compromised or killed. Instead, they have concluded they have gone to ground amid more aggressive counterintelligence by Moscow, including efforts to kill spies, like the poisoning in March in Britain of a former Russian intelligence officer that utilized a rare Russian-made nerve agent.
Current and former officials also said the expulsion of American intelligence officers from Moscow has hurt collection efforts. And officials also raised the possibility that the outing of an F.B.I. informant under scrutiny by the House intelligence committee — an examination encouraged by President Trump — has had a chilling effect on intelligence collection.
Technology companies and political campaigns in recent weeks have detected a plethora of political interference efforts originating overseas, including hacks of Republican think tanks and fake liberal grass-roots organizations created on Facebook. Senior intelligence officials, including Dan Coats, the director of national intelligence, have warned that Russians are intent on subverting American democratic institutions.
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