Five GOP Senators—Marco Rubio, Tom Cotton, Richard Burr, John Cornyn and James Lankford—have written a letter to Secretary of State Rex Tillerson calling on him to kick all Cuban diplomats out of the country and, if necessary, close the U.S. Embassy in Cuba. From the Hill:

“Cuba’s neglect of its duty to protect our diplomats and their families cannot go unchallenged,” the letter reads…

“The safety of U.S. diplomatic personnel and their families posted overseas remains one of our high priorities and a shared responsibility of those nations that host U.S. diplomatic facilities,” the senators wrote. “We urge you to remind the Cuban government of its obligation and to demand that it take verifiable action to remove these threats to our personnel and their families.”

The letter comes in response to the 21 American diplomats (and five Canadians) who suffered hearing damage while staying in hotels in Havana. Last month, reports revealed the injuries to some of the diplomats was much more serious than originally believed with some Americans suffering traumatic brain injuries.

The Associated Press has a story today suggesting that U.S. authorities still aren’t confident Cuba is responsible for the attack on our personnel. Cuba’s Raul Castro has, of course, denied the allegation but it’s how he denied it that has investigators wondering.

In a rare face-to-face conversation, Castro told U.S. diplomat Jeffrey DeLaurentis that he was equally baffled, and concerned. Predictably, Castro denied any responsibility. But U.S. officials were caught off guard by the way he addressed the matter, devoid of the indignant, how-dare-you-accuse-us attitude the U.S. had come to expect from Cuba’s leaders.

The Cubans even offered to let the FBI come down to Havana to investigate. Though U.S.-Cuban cooperation has improved recently — there was a joint “law enforcement dialogue” Friday in Washington — this level of access was extraordinary…

When the U.S. has accused Cuba in the past of misbehavior, such as harassing diplomats or cracking down on local dissidents, Havana has often accused Washington of making it up. This time, although Castro denied involvement, his government didn’t dispute that something troubling may have gone down on Cuban soil.

Yesterday, in a separate story, the AP reported that there is now even some question if the attack was a sonic device or something else, though some of the victims could hear a highly localized sound:

The blaring, grinding noise jolted the American diplomat from his bed in a Havana hotel. He moved just a few feet, and there was silence. He climbed back into bed. Inexplicably, the agonizing sound hit him again. It was as if he’d walked through some invisible wall cutting straight through his room…

In several episodes recounted by U.S. officials, victims knew it was happening in real time, and there were strong indications of a sonic attack.

Some felt vibrations, and heard sounds — loud ringing or a high-pitch chirping similar to crickets or cicadas. Others heard the grinding noise. Some victims awoke with ringing in their ears and fumbled for their alarm clocks, only to discover the ringing stopped when they moved away from their beds.

The problem is that while a sonic device of some kind might cause the hearing loss and audible sounds, it could not explain the brain damage and other symptoms experienced by some victims:

“Brain damage and concussions, it’s not possible,” said Joseph Pompei, a former MIT researcher and psychoacoustics expert. “Somebody would have to submerge their head into a pool lined with very powerful ultrasound transducers.”

So the mystery remains. Sound clearly seems to explain some of the symptoms and experiences but not all. Maybe there was another type of attack being used simultaneously. Or maybe Cuba (or Russia) has a new weapon we don’t know about.