The FBI has investigated the alleged harassment of diplomatic personnel evacuated from Cuba and China when they returned to the U.S., according to a new NBC News investigation.

Four U.S. officials told NBC that the FBI has investigated instances of harassment and break-ins reported by at least six Americans who were evacuated from China and Cuba following mysterious health attacks.

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Some of the diplomatic personnel said they suspect their homes have been broken into after they found items that had been moved or tampered with, NBC reported. Others said they felt they had been followed or noted questionable activity on their cellphones.

At least one suspect gave the FBI a laptop they suspect had been tampered with.

These diplomatic personnel, alongside many others, were forced to leave Cuba and China after colleagues reported experiencing serious medical issues.

The State Department has not yet found the cause of the spate of "health attacks" against diplomats in both countries, but they have verified that multiple U.S. officials positioned in Cuba and China are presenting similar brain injuries.

Intelligence officials probing the mysterious attacks reportedly suspect Russia is the prime perpetrator, but the Trump administration says it is uncertain who is responsible.

It was reported earlier this month that microwave weapons were suspected in the attacks, in which diplomats heard painful sounds, lost their hearing, or suffered vertigo. However, the possibility that other weapons were also used has not been ruled out.

At least one diplomat in Guangzhou, China, has experienced symptoms, as have 26 in Havana.

Medical tests have indicated that all of the diplomats are presenting similar symptoms.

U.S. officials told NBC that physicians hired by the State Department to investigate the diplomats have identified what they call a "brain network disorder" that has changed the brain structure of those affected in ways that other brain diseases do not.

The FBI declined to comment for the report.