The expulsions mark another chapter in a deepening international mystery as U.S. and Cuban authorities seek to determine how and why so many U.S. diplomats came down with a range of ailments, from hearing loss to minor brain damage, in Havana.
Secretary of State
"This order will ensure equity in our respective diplomatic operations," Tillerson said in a statement. He said he was acting due to Cuba's "failure to take appropriate steps to protect our diplomats" in accordance with international law.
Tillerson and other officials insisted the expulsions did not reduce U.S. diplomatic relations with Havana, which were only restored in 2015 after more than half a century, and U.S. officials took pains not to accuse Cuba of causing the illnesses.
Cuban officials were angered, calling the expulsions unfair and unwarranted.
In Havana, Cuban Foreign Minister Bruno Rodriguez said Cuba "strongly protests and condemns this unfounded and unacceptable decision."
Cuba "has never perpetrated, nor will it ever perpetrate attacks of any sort against diplomatic officials or their relatives, without any exception." Rodriguez said in a statement. "Neither has it ever allowed nor will it ever allow its territory to be used by third parties with that purpose."
"We think this decision is hasty and will affect bilateral relations," said Josefina Vidal, head of American affairs for the Cuban Foreign Ministry and a key diplomat in restoring ties between the two countries.
She said Cuba had rigorously participated in finding the causes of the ailments, and the expulsions seemed to prejudge the investigation.
Some U.S. officials have speculated that the attacks are the work of a rogue Cuban operatives seeking to wreck relations with the U.S., or that of a third nation.
U.S. officials initially suspected that sophisticated acoustic devices were used to aim damaging sound waves at Americans. But no such devices were found and the U.S. Embassy in Havana no longer considers that a likely cause.
Tillerson said last week that 21 U.S. Embassy personnel had been affected, but a senior State Department official said Tuesday that another American who fell ill in January was reexamined and added to the group.
The State Department official, who briefed reporters on condition of anonymity, said the 15 Cuban diplomats had seven days to leave. U.S. officials presented Cuban authorities with a list of names Tuesday, he said.
Cuba "must take more action" in response to the attacks, the official said without specifying what action was required.
Heather Nauert, the State Department spokeswoman, said the 15 were chosen because their functions related generally to the functions of the American diplomats who had to leave Cuba. None were publicly identified.
U.S. officials refused to say how many Americans are left at the U.S. Embassy in Havana, but it's believed to be fewer than 30.
Several Republican lawmakers had sharply criticized Tillerson last week for not expelling Cuban diplomats, blaming the government in Havana for the unusual ailments.
State Department officials first disclosed what they called incidents this summer, but the announcement on Friday described them for the first time as "specific attacks" that targeted U.S. officials.
They began last year and continued as recently as August, according to U.S. officials.
Since restoring relations, Washington and Havana have cooperated on several regional issues, enjoyed increased trade and a tourism boom.
Several experts and organizations that work with Cuba said expelling diplomats would be counterproductive.
"The more we escalate tensions with Cuba, the harder it will be to solve this mystery," said Daniel Erikson, who specialized in Latin America in the Obama White House and now is managing director of Blue Star Strategies, a consulting firm. "They should be taking steps to investigate," not walk away, he said.