WASHINGTON (Reuters) – Russia is the main suspect in U.S. agencies’ investigation of mysterious illnesses in American personnel in Cuba and China, NBC News reported on Tuesday.

Evidence from communications intercepts has pointed to Moscow’s involvement during the investigation involving the FBI, CIA and other agencies, NBC reported, citing three unidentified U.S. officials and two other people briefed on the probe.

The evidence, however, is not conclusive enough for the United States to assign blame publicly to Moscow, according to the NBC report.

The FBI said it did not have a comment on the NBC report. A U.S. government source familiar with official assessments said intelligence agencies would not confirm the report.

U.S. officials said in July that they are still investigating health problems at the U.S. Embassy in Cuba, and do not know who or what was behind the mysterious illnesses, which began in 2016 and have affected 26 Americans.

State Department spokeswoman Heather Nauert told Reuters on Tuesday, “We have made no determination on who or what is responsible for the health attacks.”

Symptoms have included hearing loss, tinnitus, vertigo, headaches and fatigue, a pattern consistent with “mild traumatic brain injury,” State Department officials have said.

The State Department said in June it brought a group of diplomats home from Guangzhou, China, over concern they were suffering from a mysterious malady resembling brain injury.

Cuban officials, who are conducting their own investigation, have denied involvement.

The United States believes sophisticated electromagnetic weapons may have been used on government workers, possibly in conjunction with other technologies, NBC reported.

The U.S. military has been trying to reverse-engineer the weapon or weapons used to harm the diplomats, including by testing various devices on animals, NBC said, citing Trump administration officials, congressional aides, and others.

Part of the work is being done at the directed energy research program at Kirtland Air Force Base in New Mexico, where the military has giant lasers and laboratories to test high-power electromagnetic weapons, including microwaves, NBC said.

(Reporting by Doina Chiacu, Lesley Wroughton, Mark Hosenball; Editing by Susan Thomas and Dan Grebler)