The Defense Department has spent more than a year testing a device purchased in an undercover operation that some investigators think could be the cause of a series of mysterious ailments impacting US spies, diplomats and troops that are colloquially known as Havana Syndrome, according to four sources briefed on the matter.

A division of the Department of Homeland Security, Homeland Security Investigations, purchased the device for millions of dollars in the waning days of the Biden administration, using funding provided by the Defense Department, according to two of the sources. Officials paid “eight figures” for the device, these people said, declining to offer a more specific number.

The device acquired by HSI produces pulsed radio waves, one of the sources said, which some officials and academics have speculated for years could be the cause of the incidents. Although the device is not entirely Russian in origin, it contains Russian components, this person added.

Officials have long struggled to understand how a device powerful enough to cause the kind of damage some victims have reported could be made portable; that remains a core question, according to one of the sources briefed on the device. The device could fit in a backpack, this person said.

CNN was not able to learn where — or from whom — HSI purchased the device.

It was also not clear how the US government learned of the existence of the device in order to purchase it. Havana Syndrome — and its cause — have remained frustratingly opaque to both the intelligence community and the medical community.