Russia is the main suspect of attacks against Americans in Cuba and China, NBC News reported Tuesday.

Five unnamed sources with information about the still-ongoing investigation told NBC News that while Russia is the most likely candidate, there is not enough evidence yet for the U.S. to formally accuse the nation.

“The investigation is ongoing. We have made no determination on who or what is responsible for the health attacks,” State Department spokeswoman Heather Nauert told NBC News.

The attacks targeting U.S. diplomats began in 2016, but the State Department is still not certain what kind of weapon caused the damage. Many initially believed it was a sonic attack, but experts have since determined the sound waves could not cause the damage. Experts are unsure what other weapons may have been used in conjunction.

Victims were afflicted with brain injuries including headaches, nausea and some vision issues.

Laura Kelly contributed to this article.

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