WASHINGTON — President Trump said on Monday that he blamed Cuba for the mysterious attacks that sickened American diplomats there and prompted the abrupt withdrawal of United States embassy personnel from Havana. His remarks were a direct contradiction of the State Department, which has said repeatedly it does not know the cause of the illnesses, or who is responsible.

Mr. Trump’s statement came about two weeks after his administration ordered the expulsion of 15 diplomats from the Cuban Embassy in Washington. The State Department attributed that move to what it said was Cuba’s failure to protect American diplomats stationed on its soil, as required under international law.

But senior officials in the department have said repeatedly in recent weeks that they have not yet determined who is carrying out the attacks or what precisely is causing the resulting illness. Many current and former officials say they doubt that Cuba is to blame.

The episode has cast a Cold War chill over the détente between Cuba and the United States. Mr. Trump has vowed to halt that initiative, eased under the Obama administration, calling it a “terrible and misguided deal” that rewarded a brutal government.