nited States and Cuban flags fly side-by-side on the roof of the Iberostar Hotel Parque Central near El Capitolio (background) in the historic Old Havana neighborhood in Havana, Cuba, March 20, 2016.

The State Department was preparing to order non-essential employees in Cuba to return to the U.S. Thursday, CBS News reported. The decision to cut all but essential employees in the embassy in Havana was made for the safety of employees and their families who have been experiencing symptoms possibly associated with sonic attacks.

The complaints from diplomats began in December 2016 and have continued since with 21 employees complaining of symptoms like headache, nausea and hearing loss among others. These employees live in housing provided by the Cuban government. Cuban officials have denied allegations that such attacks are being committed by the government and because some Canadian diplomats have experienced symptoms as well.

What is a sonic attack?

Sonic attacks are the use of sound waves that can then cause physical symptoms. The sound waves are undetectable by the human ear, usually at a frequency humans can’t pick up. The National Institutes of Health conducted a report on such attacks in 2001. They call the sound 'infrared sound' and constitute it as “inaudible sound with a frequency of <20 Hz.”

“Transmission of infrasound energy through the air is not as efficient as transmission through mechanical vibrations at infrasound frequencies,” according to the NIH. Such weapons are sometimes users for riot control in some countries or in war torn areas, the more serious weapons emit loud shrill noise that can be debilitating. The type of acoustic weapon that was possible used on the diplomats is currently unknown, if one was even used at all.

A travel notice is expected from officials Friday that will include travel warnings, USA Today reported. Supposedly some of the attacks took place in Cuban hotels, but no Americans traveling in Cuba have reported suffering from such attacks while traveling.

Symptoms typically present physically and seemingly unexplained. They can include nausea, headaches, hearing loss, dizziness and an overall sick feeling. While some symptoms go away the hearing loss can be permanent.

In August, State Department spokesperson Heather Nauert, gave brief information about the attacks at a news conference. “So some U.S. Government personnel who were working at our embassy in Havana, Cuba on official duties – so they were there working on behalf of the U.S. embassy there – they’ve reported some incidents which have caused a variety of physical symptoms,” she said. “I’m not going to be able to give you a ton of information about this today, but I’ll tell you what we do have that we can provide so far.”