On Tuesday, the State Department confirmed that two more Americans have fallen victim to an ongoing series of mysterious attacks targeting diplomats in Cuba, the Associated Press reports. The new cases bring the total of Americans affected by the assaults to 21.

US authorities first acknowledged the attacks in August, about nine months after diplomats began reporting bizarre sonic experiences and a puzzling spectrum of symptoms, from brain injuries to hearing loss. Despite an international investigation into the attacks, which have also affected Canadian diplomats, authorities and scientists are still baffled as to what kind of weapon or devices could have been used—let alone by whom.

The victims report a range of symptoms including dizziness, nausea, headaches, balance problems, ringing in the ears (tinnitus), nosebleeds, difficulty concentrating and recalling words, permanent hearing loss, and speech problems. There have also been brain injuries, including swelling and concussion.

Some said they felt vibrations during their attack. Others heard noises, described as either a loud, high-pitch chirping like cicadas or a jarring grinding noise. Some victims reported waking up in bed with a ringing in their ears—which stopped when they moved away from their beds. Still, others said they felt and heard nothing before their symptoms began.

Some say they were attacked in their homes, while others were attacked in the newly renovated Hotel Capri in Havana. The attacks seemed to strike at night, with some saying they came in one-minute bursts.

The combination and severity of injuries has scientists stumped. Sonic devices could be developed that could create a targeted beam of sound capable of injuring a person’s hearing. But such a device would likely be quite large and hard to conceal, the AP reports. And it’s unlikely that a single nefarious gadget could cause the broad range of symptoms that victims are reporting.

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

In May, US officials expelled two Cuban diplomats to protest the country’s failure to protect Americans there. But US officials have been cautious about accusing the Cuban government of the attacks. They’ve left open a range of possibilities, including a third country's involvement or a rogue faction. The fact that Canadian diplomats were also affected is puzzling, because Canada has kept friendlier connections in the country than the US.

“The investigation into all of this is still under way. It is an aggressive investigation,” State Department spokeswoman Heather Nauert told the AP Thursday. “We will continue doing this until we find out who or what is responsible for this.”