It started five years ago - employees at the US Embassy in Havana suffered from mysterious symptoms such as dizziness, headaches and difficulty concentrating.

In addition, several of them had been plagued by loud and loud noises before the problems broke out.

Several agencies have since investigated what has come to be known as the Havana syndrome.

In recent times, the theory that foreign powers have attacked personnel with microwave weapons has become dominant.

Secret report: Crickets were behind

Recently, a report from the US State Department went public.

In it, a research group states that the sounds that the embassy staff in Havana experienced, and in several cases recorded, in all probability come from a particularly loud cricket.

The same report dismisses the theory of microwave weapons as unlikely.

One of those who questioned the weapon theory is Robert Baloh, a professor of neurology at the University of California, UCLA.

Microwaves should have caused tissue damage and brain damage has not been detected in examinations with a magnetic camera, he says.

There is also no other evidence to support the thesis.

- Many have searched: the FBI, the CIA, the Ministry of Defense and the Cuban government.

Not a single piece of evidence for a weapon has been presented after five years of searching, he says.

- It simply does not go together.

Researcher: "Mass hysteria" probable explanation

The most likely cause of the phenomenon is, according to Baloh, mass psychogenic disease - formerly known as mass hysteria.

In short, the idea of ​​attacks is spread like a virus - and the idea can both cause symptoms and be used as an explanation for symptoms that have other causes.

- You develop illness because you think you have been exposed to something dangerous, says Robert Baloh.

Cases all over the world

Since 2016, more American officials, both inside and outside the country's borders, have been affected by the Havana syndrome.

In the past month, cases have been reported in a number of countries, including Germany and Colombia.

- I think the explosion in the number of cases is due to the fact that the Ministry of Foreign Affairs has emailed people all over the world and urged them to pay attention to certain sounds and symptoms.

This is usually how it is spread, says Robert Baloh.