More details are beginning to emerge about the mysterious “sonic attacks” that have afflicted nearly two dozen American diplomats - and a handful of Canadians - serving in Havana.

The bizarre attacks, which somehow managed to slip beneath the mainstream media radar until August when CBS published a report about the phenomenon, were first documented late last year, and continued through the spring – though one attack was reported as recently as Sept. 1. Since then, the US government has kept quiet, refusing to discuss potential suspects, or theories about how the attacks were carried out. Until now, as the Telegraph publishes new details about where the attacks occurred, as well as some insights into the government's thinking.

The Cuban government has denied involvement in the incidents; however, the US abruptly and quietly expelled two Cuban diplomats from Washington last month, shortly before CBS ran its story, raising suspicions that the US was, in fact, looking into a Cuban government role.

Yesterday, the State Department revealed that two more diplomats had exhibited symptoms associated with the attacks, bringing the total number of Americans affected to 21.

“Late last year, reports began surfacing that US diplomatic personnel in Cuba were beginning to experience strange and inexplicable symptoms including dizziness, nausea, memory lapses, difficulty hearing and loss of balance. Initial reports attributed the symptoms to a “acoustic attack,” but neglected to provide any salient details about the specific nature of the attacks, or – more importantly – who might have carried them out. The Cuban government quickly denied any knowledge of the attacks.”

Now, the Telegraph has revealed some new information about the attacks, but the details provide little precious little insight into who carried out the attacks, and how.

As one former CIA official says: "It's just mystery after mystery after mystery."

“On Thursday it emerged that the “attack” – which Cuba denies – was in one case a blaring, grinding noise which jolted an American diplomat from his bed in The Capri, a Havana hotel popular with tourists. The diplomat, according to AP, moved just a few feet, and there was silence. He climbed back into bed – and then the agonising sound hit him again. "None of this has a reasonable explanation," said Fulton Armstrong, a former CIA official who served in Havana long before America re-opened an embassy there. "It's just mystery after mystery after mystery."

According to the Telegraph, the Cuban authorities earlier this month allowed teams from the FBI and Royal Canadian Mounted Police to investigate, but no evidence has been found – at least, nothing that’s been shared with the public.

At least one source who spoke with the Telegraph thinks the Cubans are innocent.

"Had they thought the Cuban government was deliberately attacking American diplomats, that would have had a much more negative effect," said Mark Feierstein, who oversaw Barack Obama’s defrosting of Cold War-era relations while he sat on the national security council. "We haven't seen that yet."

There may be a good reason the government has been reticent about the issue: US intelligence agencies don’t know what’s going on. According to the Telegraph, the symptoms of the attacks have confounded the FBI.

Some of the diplomats effected have been diagnosed with minor brain injuries, something unlikely to result from a sound attack. Others have had problems concentrating or recalling specific words.

Investigators have tested several theories about an intentional attack by Cuba's government, a rogue faction of its security forces, or possibly a third country like – wait for it - Russia. Another theory, according to the Telegraph, is that an eavesdropping device may have gone awry. Though given the severity of the symptoms and number of victims, that seems unlikely.

Perhaps, now that previously unreleased details regarding the incident and the government’s response are beginning to leak to the press, a representative of the US’s vast intelligence apparatus will soon provide the long-overdue update the public deserves.