WASHINGTON (NewsNation Now) — A mysterious set of symptoms referred to collectively as “Havana Syndrome” is sickening U.S. personnel, mystifying the government as investigators try to determine if a foreign adversary is behind it.

Here’s what we know about Havana Syndrome:

What is it?

People who are believed to have been affected have reported headaches, dizziness and symptoms consistent with concussions, with some requiring months of medical treatment. Some have reported hearing a loud noise before the sudden onset of symptoms.

Why is it named that?

The problem has been labeled the “Havana Syndrome,” because the first cases affected personnel in 2016 at the U.S. Embassy in Cuba.

How many people have had this problem?

At least 130 cases across the government are now under investigation, up from several dozen last year, according to a U.S. defense official who was not authorized to discuss details publicly.

Why the cause for alarm?

Particularly alarming are revelations of at least two possible incidents in the Washington area, including one case near the White House in November in which an official reported dizziness.

What’s being done to stop this?

The National Security Council is leading the investigation.

Defense and intelligence officials have publicly promised to push for answers and better care for people with symptoms. Lt. Col. Thomas Campbell, a Defense Department spokesman, said the causes of any incidents “are areas of active inquiry.” Officials have not identified a suspected country, though some people affected suspect Russian involvement.

CIA Director William Burns testified before Congress that he would make the investigation “a very high priority to ensure that my colleagues get the care that they deserve and that we get to the bottom of what caused these incidents and who was responsible.”

Burns receives daily updates on the investigation, which covers employees who have reported cases this year. He has met with those reporting injuries as have other top CIA officials. The agency has worked to reduce the wait time for its employees to receive outpatient treatment at Walter Reed National Military Medical Center.

The CIA also replaced its chief medical officer with a doctor seen internally as more sympathetic to possible cases.