Context: US-Cuba relations under the Trump presidency
The US embassy in Havana was opened in 2015 as a part of Obama administration's policy to thaw frozen relations with Cuba. President Trump had recently announced a partial rollback of Obama's Cuba policy, although he had stated he wouldn't close the embassy in Havana.
Tillerson termed the symptoms experienced by diplomats, "a very serious issue with respect to the harm that certain individuals have suffered." These mysterious symptoms ranged from nausea, dizziness, headaches and hearing loss to even mild brain trauma.
President Raul Castro personally assured the US officials that Cuba wasn't involved in any way. A statement from the Cuban foreign ministry stated, "Cuba would never allow the Cuban territory to be used for any kind of action against accredited diplomats or their families."