The United States is yanking more than half its diplomatic personnel from the embassy in Havana and warning Americans not to visit Cuba, saying Friday it is for their own safety until investigators determine what caused a mysterious string of attacks that have harmed at least 21 Americans stationed there.

Senior State Department officials said U.S. diplomats have been “targeted” for “specific attacks,” a significant change from previous characterizations of what happened as simply “incidents.”

Though no one has been able to determine how at least 21 U.S. diplomats were targeted and injured over the past year, their conditions have created the biggest crisis in U.S.-Cuba relations since they were normalized by President Obama in mid-2015. Even without a perpetrator, a motive or a modus operandi identified yet, some suspect poisoned relations were the ultimate aim.

Ben Rhodes, Obama’s deputy national security adviser who negotiated renewed ties with Cuba, tweeted, “Goal of whoever is behind attacks seems to be sabotaging US-Cuba relations. Would be a shame if they succeed. Cuban people wld suffer most.”

Josefina Vidal, the top Cuban official managing relations with the United States, issued a statement reiterating assurances that Foreign Minister Bruno Rodriguez gave Secretary of State Rex Tillerson on Tuesday when he flew to Washington to explain measures Cuba has taken to protect U.S. diplomats and their families — steps Tillerson evidently found wanting.

“We consider that the decision announced by the Department of State is hasty and that it will affect the bilateral relations, specifically, the cooperation in matters of mutual interest and the exchanges on different fields between both countries,” Vidal said.

State Department officials have said Cuba has cooperated in facilitating an FBI investigation, and the Cuban government has denied having anything to do with the injuries. The State Department has shied away from pinning the blame on Havana. Among the possibilities being explored is that agents acting on behalf of a third country may be responsible.

President Trump weighed in Friday, telling reporters: “Some very bad things happened in Cuba. They did some bad things.” It was not clear whether by saying “they,” Trump was blaming Cuba. The White House did not immediately respond to a request seeking clarity on the president’s remarks.

Some of the diplomats were injured in at least one hotel in the Cuban capital, the Capri near the embassy. Employees temporarily deployed to the mission were staying there. The officials said they know of no other guests or hotel employees who reported symptoms from an attack, but concern that others might be hurt prompted them to issue a broader warning advising against travel to Cuba.

“We have no reports that private U.S. citizens have been affected, but the attacks are known to have occurred in U.S. diplomatic residences and hotels frequented by U.S. citizens,” said Secretary of State Rex Tillerson in a statement. “The Department does not have definitive answers on the cause or source of the attacks and is unable to recommend a means to mitigate exposure.”

Diplomats began complaining of a wide variety of maladies beginning late last year. New symptoms have continued to crop up, most recently in August. No Cuban employees of the embassy have reported having health problems, only Americans.

The compound of the United States embassy stands in Havana. (Desmond Boylan/AP)

Among their problems are hearing loss, dizziness, tinnitus, balance problems, visual difficulties, headaches, fatigue, cognitive issues and sleeping difficulties.

Investigators are looking into the possibility that the embassy employees were subjected to some sort of “sonic attack,” among other theories. It is not clear why American diplomats and a handful of Canadian envoys and their families would be the only ones to report symptoms.

The Canadian Embassy in Washington said Ottawa is monitoring the situation and is investigating the cause. But it said there is no reason to believe more Canadians could be affected, and there are no plans to change travel advice or remove staff from Cuba.

The decision to draw down the embassy to skeletal levels does not signify any change in U.S.-Cuban relations, State Department officials insisted. Bilateral meetings will continue, but they will have to be in the United States because U.S. diplomats will not be allowed to go to Cuba. Only people involved in the investigation or critical to the embassy and national security will be granted permission to go.

But it is expected to drive a wedge between the countries, as the Trump administration works to reverse the rapprochement that occurred under President Obama, normalizing relations after nearly 50 years of enmity, by reimposing limits on American visitors and trade unless democratic reforms are made.

Some who favor stronger U.S.-Cuban ties, contend poisoned relations were not just a byproduct, but a goal.

“Whoever is doing this obviously is trying to disrupt the normalization process between the United States and Cuba,” Sen. Patrick J. Leahy (D.-Vt) said in a statement Friday. “Someone or some government is trying to reverse that process.”

James Williams, the head of Engage Cuba, a coalition of business groups, urged redoubled efforts to solve the mystery. “We must be careful that our response does not play into the hands of the perpetrators of these attacks,” he added.

The American Foreign Service Association, the union that represents diplomats, earlier this week came out against withdrawing diplomats. Barbara Stephenson, president of the group, said diplomats commonly brave risks like illness, war and oppressive smog.

“We decide we’re going to take risks because our presence matters,” she said Friday. “This is the nature of the work that we do.”

The withdrawal order applies to all nonessential staff and their families. Only “emergency personnel” will stay. The skeletal staff is being kept to assist U.S. citizens in Cuba who have pressing issues, but more routine diplomatic and consular functions will likely be slowed.

With few staff, however, no visas will be processed at the embassy because there will not be enough people to do the work. That will hamper efforts by Cuban Americans to bring relatives to the United States.

Sen. Marco Rubio (R-Fla.) has urged Tillerson to expel all Cuban diplomats from the United States, and on Friday bemoaned that none had been sent packing. He tweeted that it was “Shameful that @StateDept withdraws most staff from @USEmbCuba but Castro can keep as many as he wants in U.S.”

The U.S. travel warning almost certainly will take a bigger bite out of Cuba’s burgeoning tourism industry. The Cuban government says more than 4 million visitors pumped almost $2 billion into the economy last year.

About 615,000 were Americans, a 34 percent increase in the first year after diplomatic relations were restored. That includes 330,000 Cuban Americans visiting relatives. The rest were Americans who fit into one of 12 categories the U.S. government considers legitimate for travel purposes, including “educational” reasons cited by many individual travelers.

Nick Miroff contributed to this report.