U.S. intelligence agencies are considering Russia the ‘prime suspect’ behind the mysterious ailment that has so far plagued 26 members of the U.S. embassies in Cuba and China since 2016, three U.S. officials and two others briefed on the investigation told NBC News.

This suspicion is backed by evidence from communication intercepts amassed during a lengthy and ongoing investigation involving the FBI, CIA, and other U.S. agencies. No further information on the nature of the intelligence was given.

The evidence is not yet conclusive enough for the U.S. to formally accuse Moscow of the attacks, which are ongoing, and have resulted in a rift between the U.S. and Cuba's burgeoning diplomatic ties—something believed to have potentially been the end game from the very beginning.

The U.S. military has been working to reverse-engineer the weapon used to attack the embassy employees, according to Trump administration officials, congressional aides, and others briefed on the investigation--including testing various devices on animals. Part of that research involves the directed energy research program at Kirtland Air Force Base in New Mexico. The base has advanced laboratories used to test high-power electromagnetic weapons, including microwaves, which are now considered to be a likely candidate for the type of weapon used—although others suspect ultrasound is to blame. The U.S. is also exploring the possibility that multiple technologies were used in the attacks, possibly in conjunction with microwave radiation.

Victims of the attacks described a sudden wave of nausea, dizziness, and headaches that followed strange sounds that they compared to loud crickets or screeching metal, and have been treated for ear complaints, hearing loss, dizziness, tinnitus, balance problems, visual complaints, headaches, fatigue, cognitive issues and difficulty sleeping.

Doctors who have studied the victims’ brains found abnormalities similar to that suffered by military veterans in Iraq and Afghanistan who have survived explosions, although the soldiers' injuries--which included concussions and damage to their white matter tracts--were attributed to explosive shock waves. Doctors who examined the embassy victims earlier this year said that they had suffered concussions, but were unable to make any determination beyond that.

None of the embassy victims reported being involved in any explosions.

