"There are some people with symptoms happening that are unexplained," Sen. Jeff Flake, an Arizona Republican, told CNN on Friday during a trip to the island. "The Cubans bristle at the word 'attack.' I think they are justified at doing so. The FBI has said there is no evidence of an attack. We shouldn't be using that word."
Flake, a backer of improved US-Cuban relations and a member of the Senate Committee on Foreign Relations, which is holding the hearing, said he had received classified briefings that backed up Cuba's denials of involvement in the incidents.
Critics of the Cuban government said they must know more than they are letting on about what happened.
"It is impossible to conduct 24 separate & sophisticated attacks on U.S. Govt personnel in #Havana without the #CastroRegime knowing about it," Sen. Marco Rubio, a Florida Republican, shot back to Flake on Twitter. "Any U.S. official briefed on matter knows full well that while method of attack still in question, that attacks & injuries occurred isn't."
US officials tell CNN they are still mystified by the incidents that led the US to withdraw most of its personnel from the embassy in Havana and issue a travel warning for people thinking of visiting the communist-run island.
Rubio, a Cuban-American and dogged critic of the Cuban government, will lead the Senate Foreign Relations committee hearing, which will look into the unexplained illnesses that befell US diplomats and their family members and the State Department's response.
Starting in November 2016, US government employees began to experience strange symptoms, often late at night and sometimes accompanied by sounds similar to insects chirping or metal grinding, US officials told CNN.
The diplomats found that if they left the rooms they were in, the symptoms and sounds immediately stopped, suggesting to them that they were being targeted by an incredibly precise device, possibly a sonic weapon.
Diplomats have been treated for concussion-like symptoms, including hearing loss, dizziness, balance problems, visual complaints, headaches, fatigue, cognitive issues and difficulty sleeping, US officials said.
Canadian diplomats and family members reported similar ailments.
While Cuban officials have vigorously denied any involvement in the incidents, President Donald Trump said in 2017 that the Cubans were "responsible" for the diplomats falling ill.
FBI agents have traveled to Havana to investigate the incidents but so far have not determined what caused the mystery illnesses.
On Monday, a State Department spokesperson told CNN that Secretary of State Rex Tillerson has decided whether to form an Accountability Review Board to look into the security incidents and would announce that decision shortly, pending congressional notification.
The Accountability Review Board is an internal State Department mechanism to review security incidents involving diplomatic personnel. The most well-known recent review board followed the deadly attack on a diplomatic compound in Benghazi, Libya, in 2012.
The Miami Herald was first to reveal the Cuba development, also reporting Monday
that the State Department had delayed the publication of a medical article on the diplomats' affliction, citing national security concerns.
"As Secretary Tillerson has stated, we will not share information that violates individuals' privacy or reveals their medical conditions," the State Department told CNN. "He also emphasized that we will not release information that helps the perpetrator determine the effectiveness of any attacks."
Tillerson had told reporters at a news conference last month that the US is "convinced these were targeted attacks."
While Tillerson has not accused the Cuban government of being behind the incidents, he said he holds the government responsible for allowing them to continue.
"What we've said to the Cubans is [it's] a small island, you got a sophisticated security apparatus, you probably know who's doing it, you can stop it," Tillerson said in December. "It's as simple as that."
As the investigation into the incidents continues, the US Embassy in Havana is no longer issuing visas to Cubans and has reduced the services it provides to Americans visiting the island.
When news of the incidents first emerged, the US expelled two Cuban diplomats from their embassy in Washington. That action was followed by the expulsion of 15 Cuban diplomats in October.
"We understand the Cubans don't like the actions we've taken," Tillerson said at the news conference last month. "We don't like our diplomats being targeted."
"This is an important year in Cuba. There is a transition going on in terms of leadership," Flake told CNN. "We ought to be here in full force, but go to the US Embassy and there's just a skeletal staff."