Top Republican senators pressed the Trump administration on Friday to expel Cuban diplomats and consider closing the U.S. embassy in Havana over the mysterious “acoustic” attacks on Americans stationed on the island.

“Cuba’s neglect of its duty to protect our diplomats and their families cannot go unchallenged,” the Republican senators said in a letter to Secretary of State Rex Tillerson.

They urged the State Department to “immediately declare all accredited Cuban diplomats in the United States persona non grata and, if Cuba does not take tangible action, close the U.S. Embassy in Havana.”

The attacks, which generally have occurred at night, have confounded U.S. officials.

Victims – there are at least 21 Americans with symptoms – have reported hearing loud noises or feeling vibrations before suffering a range of aftereffects. This includes everything from hearing loss to speech problems to concussions.

Suspicion initially focused on a sonic weapon, and on the Cubans. Yet the diagnosis of mild brain injury, considered unlikely to result from sound, has stumped the FBI, the State Department and U.S. intelligence agencies involved in the investigation.

Some victims now have problems concentrating or recalling specific words, several officials told The Associated Press, the latest signs of more serious damage than the U.S. government initially realized. The United States first acknowledged the attacks in August — nine months after symptoms were first reported.

The U.S. already has expelled two Cuban diplomats to protest the communist government’s failure to protect Americans serving there. But the U.S. has taken pains not to accuse Havana of perpetrating the attacks. It’s a sign investigators believe that even if elements of Cuba’s security forces were involved, it wasn’t necessarily directed from the top.

But the attacks have revived criticism on Capitol Hill of the Obama-era decision to restore diplomatic ties with Cuba. The letter to Tillerson was signed by Sens. Tom Cotton, R-Ark.; Richard Burr, R-N.C.; John Cornyn, R-Texas; Marco Rubio, R-Fla.; and James Lankford, R-Okla.

“Our officials and their families have been the targets of unacceptable levels of harassment and ‘acoustic’ attacks that, in some cases, have caused permanent hearing damage and other significant injuries,” they wrote.

“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. 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.”

Asked about the attacks on Thursday, State Department spokeswoman Heather Nauert confirmed that 21 people have been affected, leaving open the possibility that number could rise.

“They continue to undergo tests,” she said, noting that personnel “are able to leave Havana, leave Cuba, and return back home if they wish.”

“The investigation into all of this is still underway. It is an aggressive investigation that continues, and we will continue doing this until we find out who or what is responsible for this,” she said.

The Associated Press contributed to this report.