WASHINGTON — Intelligence agencies investigating mysterious "attacks" that led to brain injuries in U.S. personnel in Cuba and China consider Russia to be the main suspect, three U.S. officials and two others briefed on the investigation tell NBC News.

The suspicion that Russia is likely behind the alleged attacks is backed up by evidence from communications intercepts, known in the spy world as signals intelligence, amassed during a lengthy and ongoing investigation involving the FBI, the CIA and other U.S. agencies. The officials declined to elaborate on the nature of the intelligence.

The evidence is not yet conclusive enough, however, for the U.S. to formally assign blame to Moscow for incidents that started in late 2016 and have continued in 2018, causing a major rupture in U.S.-Cuba relations.

Since last year, the U.S. military has been working to reverse-engineer the weapon or weapons used to harm the diplomats, according to Trump administration officials, congressional aides and others briefed on the investigation, including by testing various devices on animals. As part of that effort, the U.S. has turned to the Air Force and its directed energy research program at Kirtland Air Force Base in New Mexico, where the military has giant lasers and advanced laboratories to test high-power electromagnetic weapons, including microwaves.

Although the U.S. believes sophisticated microwaves or another type of electromagnetic weapon were likely used on the U.S. government workers, they are also exploring the possibility that one or more additional technologies were also used, possibly in conjunction with microwaves, officials and others involved in the government's investigation say.

The U.S. has said 26 government workers were injured in unexplained attacks at their homes and hotels in Havana starting in late 2016, causing brain injuries, hearing loss and problems with cognition, balance, vision and hearing problems. Strange sounds heard by the workers initially led investigators to suspect a sonic weapon, but the FBI later determined sound waves by themselves couldn't have caused the injuries.

This year, one U.S. worker in China was diagnosed with similar symptoms after hearing bizarre sounds in Guangzhou, and more from China are being tested.

View this graphic on nbcnews.com

View this graphic on nbcnews.com

The precise motive remains unclear, but the incidents have driven a wedge between the U.S. and Cuba that has led Washington to remove most of its diplomats and spies from the island. Early in the investigation, senior U.S. officials raised the possibility the illnesses were unintended consequences of some new spying technology. But the fact the incidents continued long after they became publicly known has cast doubt on the possibility that the damage was accidental.

In testimony before Congress last week, State Department officials were unanimous that the incidents should be considered "attacks."

"The State Department has come to the determination that they were attacks," Ambassador Peter Boode, who leads the task force responding to the incidents, told a House Foreign Affairs Committee panel.

A Russian-made Lada car passes by the Russian Embassy in Havana on Dec. 12, 2017. Alexandre Meneghini / Reuters file

A U.S. official separately tells NBC News that the U.S. has "no reason to believe this was anything but an intentional act."

If Russia did use a futuristic weapon to damage the brains of U.S. personnel, it would mark a stunning escalation in Russian aggression toward Western nations, compounded recently by the use of a military-grade nerve agent to poison an ex-spy and his daughter in Britain. Although the full extent of the resulting diplomatic fallout is difficult to predict, a determination that Russia was behind the Cuba attacks would trigger outrage in Congress and foreign capitals and calls for an immediate, concerted response, especially as President Donald Trump faces continued questions about his willingness to challenge Russia and President Vladimir Putin.

Russian government officials did not immediately respond to requests for comment.

In a statement, State Department spokeswoman Heather Nauert said, "The investigation is ongoing. We have made no determination on who or what is responsible for the health attacks."

The Office of the Director of National Intelligence declined to comment.

After initial publication of this story, Cuban officials blasted it as irresponsible via Twitter. Carlos de Cossio, director general of the Cuban Foreign Ministry's U.S. Department, tweeted, "Cuba does not act and does not allow anyone to act against the diplomats of any country in its territory." Cuban ambassador to the U.S. Jose Ramon Cabanas said it was "painful to see how NBC News echoes others to mount speculation without evidence, using dark sources. … Where are Cuba sources?"

On Cuban television, de Cossio said the U.S. has not produced any objective evidence that attacks of any kind occurred in Cuba, and that the "scientific community" has rejected successive U.S. theories about the source of the alleged attacks. "The way in which the United States government has dealt with this matter of the supposed health damages to its diplomats is worrisome," he said. "From the beginning, it has been handled with a high dose of political manipulation. For more than a year and a half, the State Department has launched specific accusations against Cuba without any evidence whatsoever supporting those claims and without any credibility."

The strong U.S. suspicion that Russia is behind the incidents means that Cuba's government is no longer considered the likely culprit. Still, officials did not rule out the possibility that the Cuban intelligence services may have offered the Russians some level of cooperation or tacit consent. Russian intelligence agencies operate in force in Cuba, as do Chinese spies, officials say.

Cuba's government has repeatedly denied any knowledge of or involvement in attacks on Americans in Cuba, and has argued there is no evidence the attacks even took place. The U.S. maintains that whoever perpetrated the attacks, Cuba bears responsibility for failing to protect U.S. diplomats on Cuban soil.

Although the U.S. has never disclosed their identities, officials tell NBC News that in addition to State Department diplomats, the victims include multiple CIA officers, at least one member of the U.S. military, and representatives of other agencies.