The Cuban national flag is seen raised over their new embassy in Washington on July 20, 2015. (Carlos Barria/Reuters)

The State Department will ­expel 15 Cuban diplomats after dramatically reducing the U.S. mission in Cuba last week, citing unexplained injuries among American personnel there.

The decision announced Tuesday is certain to deepen the two countries' rift over what the State Department has called "specific attacks" on U.S. diplomats during the past 10 months. The United States has not blamed Cuba, which has denied any involvement and has cooperated with FBI agents dispatched there.

Secretary of State Rex Tillerson said in a statement that the expulsion was "due to Cuba's failure to take appropriate steps to protect our diplomats in accordance with its obligations under the Vienna Convention. This order will ensure equity in our respective diplomatic operations."

[All the theories about what’s happening to U.S. diplomats in Cuba]

The expelled Cubans, whose names were on a list presented to the Cuban ambassador Tuesday morning, will have seven days to leave the country. Last week, the State Department said it would pull more than half its personnel out of the U.S. Embassy in Havana, and they are expected to leave by the end of the week.

Neither the FBI nor a separate Cuban investigation has been able to determine what and who is causing the maladies that have befallen at least 22 Americans stationed at the embassy, with symptoms ranging from hearing loss to cognitive disorders. They are believed to have been "targeted" either in their residences within compounds owned by the Cuban government or in hotels. The most recent incident happened in late August, but some conditions are still being diagnosed. On Monday, the State Department confirmed an additional victim, who is believed to have been attacked in January.

One possibility being explored is whether the diplomats were made ill by a "sonic attack," though the State Department has refrained from using that term. It is also possible that they are being singled out by a third country seeking to create tension between Cuba and the United States, which normalized relations and reopened their embassies two years ago after half a century of enmity.

[U.S. slashes embassy staff in Cuba, issuing travel warning]

Now, what had been hoped to be a historic opening is backsliding to an era of mutual suspicion and recrimination. On Friday, the State Department issued a travel warning advising all American visitors to stay away because their safety could not be guaranteed, a measure that is certain to harm Cuban tourism, the most dynamic segment of the economy. It also suspended issuing visas to Cubans, a step that makes it difficult for Cuban Americans to reunite with relatives.

"It appears that this plays right into the hands of a potential rogue actor — Russia perhaps — that is trying to create a further wedge between our two countries and other nations in the hemisphere," said Rep. Eliot L. Engel (N.Y.), the top Democrat on the House Foreign Affairs Committee.

Sen. Marco Rubio (R-Fla.), however, scoffed at the notion that Cuba is not responsible.

"No one should be fooled by the Castro regime's claim it knows nothing about how these harmful attacks are occurring or who perpetrated them," he said, calling for the embassy in Havana to be downgraded to an interests section.

But it remained unclear how expelling Cubans would further the investigation into one of the great diplomatic whodunits of all time. And the U.S. response could be playing into the hands of the perpetrators, whoever they are.

"If these health incidents are indeed targeted attacks, their intentions are to disrupt bilateral relations and affect dialogue and engagement," said Collin Laverty, head of Cuba Educational Travel. "Scaling down embassies gives them the result they are aiming for."

Cuban officials last week expressed disappointment with the U.S. reaction. A concerned Foreign Minister Bruno Rodríguez Parrilla flew to Washington in a futile attempt to dissuade Tillerson from withdrawing U.S. diplomats. But the Cubans have said they will continue to cooperate in what is already an unprecedented willingness to allow FBI agents into Cuba to investigate.

"If the U.S. government is serious about solving this mystery, they shouldn't make it more difficult to cooperate with the Cuban government during this critical time of the investigation," said James Williams, president of Engage Cuba, a group that aims to strengthen U.S.-Cuba ties.

The 15 Cuban diplomats being expelled represent about the same percentage being withdrawn from the U.S. Embassy, officials said, though the actual number of personnel ordered to leave Havana has not been disclosed.

The Americans will not be allowed to return to Cuba until the government in Havana can ensure their well-being, said the U.S. official assigned to brief reporters on the expulsion. He could not, however, explain how the Cubans could offer those assurances if no one knows the cause, much less when it may be safe for diplomats to return.

"We are making it clear that the safety and well-being of our people is being affected by these health attacks," said a State Department official, speaking on the condition of anonymity under administration ground rules for briefing the media. "We can no longer expose them to the environment down there."