An investigation by the FBI, CIA and other agencies has unearthed communications intercepts that US officials say point the finger of suspicion at the old Cold War enemy.
However, the three unidentified officials as well as two other people briefed on the probe, told NBC the evidence was not strong enough for the US to publicly blame Moscow.
US officials said in July they were still investigating health problems at the US embassy in Cuba, and do not know who or what was behind the mysterious illnesses, which began in 2016 and have affected 26 Americans.
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 also 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 any involvement or knowledge of what was behind it.
The United States believes sophisticated electromagnetic weapons may have been used on government workers, possibly in conjunction with other technologies, NBC reported.
The US military has been working on measures that can counteract the effects of 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 programme 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.
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