A number of U.S. diplomats stationed in Guangzhou, China, were evacuated Wednesday over fears they were the target of so-called sonic attacks, similar to the phenomenon that hit U.S. officials in Cuba last year, the State Department said.

Foggy Bottom issued a medical alert last month after one official suffered mild traumatic brain injury, sparking fears that U.S. government personnel in China were being targeted using the same methods that forced 24 U.S. officials to flee the Caribbean island in 2017.

The State Department started a screening program for employees and their families last month out of the consulate in Guangzhou, and as a result several have been sent home for more tests.

“The department has sent a number of individuals for further evaluation and a comprehensive assessment of their symptoms and findings in the United States,” spokeswoman Heather Nauert said Wednesday.

The New York Times reported earlier that two officials had been sent back to the U.S.

“Medical professionals will continue to conduct full evaluations to determine the cause of the reported symptoms and whether the findings are consistent with those noted in previously affected government personnel or possibly completely unrelated,” Nauert added.

One of the officials flown back to the U.S. was Mark Lenzi, a security engineering officer at the Guangzhou consulate. In an interview with the New York Times, Lenzi described the sounds he heard as marbles rolling around a metal funnel.

Lenzi and his wife said over the course of 2017 they experienced headaches, sleeplessness and nausea, and heard odd noises, but they did not connect it to the “sonic attacks” until last month when the U.S. government issued the alert.

Secretary of State Mike Pompeo said Tuesday that an investigation had been opened to examine “the unexplained health incidents that have affected a number of U.S. government personnel and family members stationed overseas.”

The attacks began more than 18 months ago, yet intelligence operatives still have no credible explanation for what is happening. Some U.S. officials suspect Beijing and Moscow may be involved.

Here’s a timeline of the key events so far:

November 2016: Diplomats stationed in Havana began reporting hearing strange sounds that targeted specific individuals. The victims reported hearing loss, dizziness, tinnitus, balance problems, visual difficulties, headaches, fatigue, cognitive issues and sleeping difficulties.

Diplomats stationed in Havana began reporting hearing strange sounds that targeted specific individuals. The victims reported hearing loss, dizziness, tinnitus, balance problems, visual difficulties, headaches, fatigue, cognitive issues and sleeping difficulties. August 2017: Physicians at the University of Pennsylvania and University of Miami diagnosed 24 of 80 embassy employees with “mild traumatic brain injury,” which they said was likely caused by “trauma due to a non-natural source.”

Physicians at the University of Pennsylvania and University of Miami diagnosed 24 of 80 embassy employees with “mild traumatic brain injury,” which they said was likely caused by “trauma due to a non-natural source.” September 2017: The U.S. announces it is pulling most of its American embassy staff and their families out of Havana due to safety fears.

The U.S. announces it is pulling most of its American embassy staff and their families out of Havana due to safety fears. October 2017: The White House expels 15 Cuban diplomats from the U.S. as Donald Trump directly accuses the Cuban government of perpetrating the attacks on 24 U.S. officials, damaging already fragile relations between the two countries.

The White House expels 15 Cuban diplomats from the U.S. as Donald Trump directly accuses the Cuban government of perpetrating the attacks on 24 U.S. officials, damaging already fragile relations between the two countries. October 2017: Cuban officials dismiss as “science fiction” reports of attacks against U.S. officials, claiming that crickets were to blame.

Cuban officials dismiss as “science fiction” reports of attacks against U.S. officials, claiming that crickets were to blame. November 2017: Suspicion switches to Russia following an apparent acoustic attack on an official working for the United States Agency for International Development in Uzbekistan.

Suspicion switches to Russia following an apparent acoustic attack on an official working for the United States Agency for International Development in Uzbekistan. January 2018: An interim FBI report, seen by AP, said that after a four-month investigation, agents have found no evidence that sound waves could have damaged the Americans’ health. Then-Secretary of State Rex Tillerson said he stood behind claims of an attack.

An interim FBI report, seen by AP, said that after a four-month investigation, agents have found no evidence that sound waves could have damaged the Americans’ health. Then-Secretary of State Rex Tillerson said he stood behind claims of an attack. January 2018: Sen. Marco Rubio, who chaired the U.S. Senate Foreign Relations Subcommittee hearing into the attacks, called the weapon being used “very sophisticated technology that does not exist in the U.S. or anywhere else in the world” — despite no evidence that these weapons exist.

Sen. Marco Rubio, who chaired the U.S. Senate Foreign Relations Subcommittee hearing into the attacks, called the weapon being used “very sophisticated technology that does not exist in the U.S. or anywhere else in the world” — despite no evidence that these weapons exist. May 2018: The State Department confirms one official based in China suffered mild brain trauma after experiencing “subtle and vague, but abnormal, sensations of sound and pressure.”