Dizziness. Ringing of the ears. Piercing sounds that suddenly disappear when a door is opened. These are just some of the mysterious symptoms reported by American diplomats in Cuba starting in 2016.

Since then, continuing investigations led by intelligence agencies, scientists and the State Department have become an intricate "whodunnit" with widely varying accounts of the possible causes — and perpetrators — of what were almost immediately labeled as attacks against U.S. personnel.

Officially, the State Department maintains it is still investigating the matter. NBC has reported that officials within US intelligence agencies believe that Russia is to blame.

Dizzy in paradise

Two years after the re-opening of the U.S. Embassy in Havana, the Department of State opened an investigation into what were quickly labeled as "health attacks" against embassy personnel Getty Images/Chip Somodevilla In 2017, the State Department began investigating a string of complaints from diplomats in Havana. Their symptoms ranged from dizziness and headaches to cognitive disfunction. Those affected also said the symptoms began during or after they heard sounds, similar to those made by cicada bugs, while in their homes or hotel rooms, and felt pressure effects similar to those experienced in a moving car with the windows slightly open.

The DOS review showed signs of "neurotrauma," but was otherwise inconclusive. A subsequent Department of Defense-sponsored inquiry reported no indications of brain injury, as some experts had suspected.

The Pentagon-sponsored study points to three possible methods which may have been used to disorient the diplomats, but caution that an actual proclamation would be "foolhardy." Perhaps the only solid finding is that drugs and/or toxins were not responsible — although even this comes with a caveat, as an introduced chemical could have been used to exacerbate the effects of an ultrasonic, electromagnetic, or even microwave technology.

Although each of the studies has reported disparate and sometimes contradictory information, all the experts seem perplexed by this Sherlock Holmes-like mystery. While they continue the process of elimination, none have yet to proclaim the likely perpetrator — at least, not officially.

It's not the rum, it's the Russians

On Sep. 29, 2017, then-Secretary of State Rex Tillerson ordered the departure of non-emergency embassy personnel assigned to Havana, and concurrently issued an official travel warning to US citizens. Tillerson said in an official statement that the move was not an indictment against Cuba, and the country has continued to deny involvement in the "health attacks" against Americans.

Russian President Vladimir Putin. NBC reported today that U.S. officials believe Russia is behind the "health attacks" against Americans in Cuba. Getty Images/Adam Berry Officially, US officials are unwilling to assign blame. But NBC reported yesterday that unofficially, officials with the intelligence agencies conducting the investigation believe Russia is behind the attacks. The officials, who were unnamed in the NBC report, reportedly point to "signals intelligence"— typically referring to intercepted communications like emails, phone calls and web chats — collected during the investigation.

The NBC account marks the first time a US official has implicated the Russians, even anonymously. But suspicions of Russian involvement were raised late last year when, according to a CBS report, an official at the American embassy in Uzbekistan reported an attack similar to those in Cuba. Maria Zakharova, a spokeswoman for the Russian Foreign Ministry, said accusations of Russian involvement were "absurd," and even offered to assist with the investigation in Cuba.

While U.S. officials have provided no definitive answers regarding the primary suspects in this mystery, there are some precedents to raise suspicions of a foreign adversary wielding sound as a weapon.

Sound warfare

Using sound technologies to disrupt or disorient a target is not an unusual tactic, especially to confuse or terrify enemies.

US forces, for example, use long-range accoustic devices for a number of purposes, including deterrence and defense. The devices are useful in determining and clarifying the intentions of an approaching unit or vessel at sea, and deterring potential combatants as they approach.

In another instance, the International Red Cross reported that U.S. military personnel at Guantanamo Bay sometimes used music as one form of "ill treatment." Its 2007 report alleges the detainees were forced to listen to loud music and other disturbing sounds for extended periods, often causing sleep deprivation. A Department of Defense spokesperson told Politico at the time that such "sensory deprivation" techniques are not authorized for use by U.S. personnel.

Given the unconventional history of weaponized sound, the "attacks" on Americans in Havana may be a sign of advancements in sound warfare, but most of the investigation's details are still under wraps.

In June, State Department personnel in Guanzhou, China reported symptoms matching those from the Cuban incidents. The affected personnel were evacuated, and then-Secretary Tillerson announced the department was expanding its investigation.