Sixteen American diplomats and relatives now confirmed to have suffered in Cuban 'sonic device' mystery

More than a dozen Americans working at the US embassy in Havana have suffered symptoms from a suspected “sonic device”, the state department has confirmed – significantly increasing the scale of the scare that has baffled experts.

The state department first revealed their concerns on August 9, when asked by a reporter at a daily press briefing.

Heather Nauert, the spokesman for the department, would only say that there had been an “incident” involving an unknown number of officials. Some of them, she said, had returned to the US for treatment.

Earlier this week it was confirmed that several suffered brain damage. And on Thursday the department said that 16 Americans had been affected.

"We take this situation extremely seriously," said Ms Nauert, adding that the investigation into the incident is continuing.

Cuba has denied involvement in the incidents, and said this month that it is investigating the US allegations. The state department has not blamed Cuba for the attacks, but did ask two Cuban diplomats to leave Washington in May.

It is now known that the officials reported being unwell between autumn last year and spring of this year.

Some suffered from a loss of hearing, and CBS reported this week that the diplomats and their families had been diagnosed with nausea, headaches and balance disorders. Some also had conditions as serious as mild traumatic brain injury and damage to the central nervous system.

Canada's government has said at least one Canadian diplomat in Cuba also had been treated for hearing loss.

But many unanswered questions remain.

The devices are believed to be inaudible, high frequency devices placed near the residences of the diplomats.

American officials would be aware of the potential for harassment and eavesdropping, but the use of sonic devices is highly unusual.

Furthermore, the fact that a Canadian diplomat was also affected muddies the waters: Canada, unlike the US, has a long history of friendly relations with Cuba.

British officials are not believed to have been affected.

Experts are also puzzled over the use of an inaudible device, pointing out that if the aim was intimidation you would need the targets to know they were in the cross-hairs.

The use of audio devices is in itself not uncommon, however - the Israeli army has a device known as "the screamer" that causes nausea and dizziness, and American police have used sound cannons to control crowds in the aftermath of Hurricane Katrina in New Orleans and during the G20 meetings in Pittsburgh.

Some observers believe the harmful devices could be eavesdropping machines which have malfunctioned.

James Lewis, a former US diplomat who now works as an analyst at the Center for Strategic and International Studies, said that if the surveillance equipment was "misconfigured, it could produce inaudible noise."

He suggested that Cuban intelligence agents may have set up surveillance on American communications "and it screwed up”.

"They'll super glue your car - the key - that's a popular one," he said. "Or they'll pop your tyres.

“If you're going to harass people, there are a lot more fun ways to do it."