UNITED NATIONS -- U.S. diplomats serving in Cuba have been experiencing health problems, and the cause is a mystery.

It began last year -- in Havana. More than a dozen staffers at the U.S. Embassy experienced unexplained ailments that included symptoms like headaches and sleeplessness.

U.S. Embassy in Cuba CBS News

Sources say some suffered permanent hearing damage and some had to return home.

State Department spokeswoman Heather Nauert addressed the report in a Thursday press briefing.

"We don't know exactly where this came from," Nauert said. "We cant blame any one individual or a country at this point yet."

But so far, the investigation has shown that the culprit is likely a high-tech sonic device that can't be heard by humans, but clearly can be harmful.

Officials believe it was operating in or around the homes of Embassy workers.

The question is: Who put it there and why?

Vince Houghton CBS News

"It's audio but it's beyond the range of our ears," said Vince Houghton, an intelligence historian and curator at the International Spy Museum.

He says Cuba, or even the Russians, could have been carrying out an intelligence operation that went south.

But Houghton and other intelligence experts say it could also have been a routine intimidation campaign taken to another level.

"This could be new technology that had a side effect that no one had expected," Houghton told CBS News. "On the other hand, it could have been designed ... to harass, to make people feel uncomfortable."

To retaliate, the U.S. kicked out two Cuban diplomats in May.

The Cuban government called the expulsions "unjustified" and "baseless" on state-run television Wednesday night.

The Cuban Ministry of Foreign Affairs says it "has never, nor would it ever, allow … the Cuban territory [to] be used for any action against accredited diplomatic officials or their families."

There are indications that diplomats are still being affected, and it's not just the U.S. Canada says at least one of its diplomats suffered severe hearing loss. The FBI and State Department continue to investigate.