It started in the fall of 2016.

Diplomats reported hearing loss and mild brain damage after hearing unusual and puzzling sounds at the U.S. Embassy in Cuba.

But now it's spreading to embassies in other countries, with nearly 200 people affected.

The State Department is virtually mum, other than to characterize their suspicion as unknown "sonic attacks" targeted U.S. diplomats.

Some of the strange sounds have been recorded and released to the public. The victims’ symptoms include “hearing loss, dizziness, tinnitus, balance problems, visual difficulties, headaches, fatigue, cognitive issues and sleeping problems.

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Recently, the Associated Press reported 26 Americans have been “medically confirmed” to have been injured in the wave of attacks. Diplomats who have served in at least seven cities in four countries have sought testing.

The latest incident occurred in conjunction with President Trump's recent visit with Kim Jong-Un. As Trump was heading to Singapore for the historic summit with North Korea’s leader, a State Department diplomatic security agent who was part of the advance team reported hearing an unusual sound he believed was similar to what was experienced by U.S. diplomats in Cuba and China who later became ill.

The agent immediately underwent medical screening – part of a new U.S. government protocol established to respond to such potential health incidents anywhere in the world. And while the president was flying to the Southeast Asian city state, the U.S. delegation preparing for his arrival was exchanging urgent messages with the State Department headquarters back in Washington, including the agency's Diplomatic Security and the U.S. Secret Service.

The AP reported four U.S. officials confirmed it was a false alarm, but there is a heightened awareness among security officials as the White House prepares to meet with Russian President Vladimir Putin.

Suspicions of Russian involvement in the attacks were raised last September when an unnamed USAID officer and his wife were flown out of Tashkent, Uzbekistan, for evaluation after they reported events that echoed what happened in Havana. The State Department later said it isn’t "going to discuss ‎every case individually."

“We can confirm that there was no incident in Uzbekistan,” a State Department official added.

The U.S. has issued an alert warning Americans traveling to China to seek medical attention if they experience "auditory or sensory phenomena" similar to what was reported in Havana, and the State Department recommended anyone traveling to Cuba should "reconsider" their plans.

Since reporters first blindsided spokesperson Heather Nauert with questions about the incidents last August, the State Department has been reluctant to give on-the-record details about its investigation. It still refers to these cases as "specific attacks" without saying what is specifically happening.