– In 2016, U.S. intelligence agencies delivered urgent and explicit warnings about Russia’s intentions to try to tip the U.S. presidential election — and a detailed assessment of the operation afterward — thanks in large part to informants close to President Vladimir Putin and in the Kremlin who provided crucial details.

But two years later, the vital Kremlin informants have largely gone silent, leaving the CIA and other spy agencies in the dark about precisely what Putin’s intentions are for November’s midterm elections, according to U.S. officials familiar with the intelligence.

The officials do not believe the sources have been compromised or killed. Instead, they have concluded they have gone underground amid more aggressive counterintelligence by Moscow, including efforts to kill spies, like the poisoning in March in Britain of a former Russian intelligence officer using a rare Russian-made nerve agent.

Current and former officials also said the expulsion of U.S. intelligence officers from Moscow has hurt collection efforts. And officials also raised the possibility that the outing of an FBI informant under scrutiny by the House intelligence committee — an examination encouraged by President Donald 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, director of national intelligence, have warned that Russians are intent on subverting U.S. democratic institutions.

But intelligence agencies have not been able to say precisely what Putin’s intentions are: He could be trying to tilt the midterm elections, sow chaos or generally undermine trust in the democratic process.

The officials, seeking to protect methods of collection from Russia, would not provide details about lost sources, but acknowledged the degradation in the information collected from Russia. They spoke on condition of anonymity.

To determine what the Russian government is up to, the United States employs multiple forms of intelligence, including intercepted communications and penetrated computer networks.

The United States continues to intercept Russian communication, and the flow of that intelligence remains strong, said current and former officials. And Russian informants could still meet their CIA handlers outside Russia, further from Moscow’s counterintelligence apparatus.

But people inside or close to the Kremlin remain critical to divining whether there is a strategy behind seemingly scattershot efforts to undermine U.S. institutions.

Spies and informants overseas also give U.S. intelligence agencies early warning about influence campaigns, interference operations or other attempts to compromise the United States. That information, in turn, can improve the ability of domestic agencies, like the Department of Homeland Security and the FBI, to quickly identify and attempt to stop those efforts.

The United States still should have a clear view of Putin’s strategies and intention to interfere in democratic elections, said Michael Carpenter, a Russia expert and former Obama administration official. He pointed to fake social media accounts created as part of Russian intelligence operations that have drummed up support for white nationalists and the Black Lives Matter movement and have supported far-right, far-left and pro-Russian candidates in the United States and in Europe.

“Clearly Russia is playing both sides of controversial issues precisely to sow chaos. But that said, it is not just chaos; there are certain candidates Russia prefers to see in office,” said Carpenter, now at the Penn Biden Center for Diplomacy and Global Engagement. “The Russians are trying to support anti-establishment and pro-Russian candidates, not just in the U.S. but everywhere.”