Washington(CNN) The mysterious sonic attack on US diplomats stationed in Cuba is raising questions about weapons that sound more like something out of James Bond than reality.
The US believes sophisticated devices that operated outside the range of audible sound were deployed either inside or outside diplomats' residences in Havana, injuring at least 16 Americans, several senior State Department officials have told CNN.
The attacks, which began in November and ended this spring, left some with mild traumatic brain injury; two might have permanent hearing loss. And the invisible assaults may be continuing against other targets, with Canadian diplomats and their family members reporting similar symptoms in June.
Experts say the question isn't just what kind of weapon might have been used, but whether the damage was caused by a weapon at all.
While a multi-agency investigation is underway, publicly available information is largely anecdotal, and not a good basis for drawing conclusions, these experts warn. They point to a number of factors that raise doubts, including the fact that sonic weapons don't produce consistent results.
Others point to the political circumstances -- the attacks happened at a time when the US and Cuba were working to improve relations in the wake of the Obama administration's decision to end the decades-long US embargo against the island nation. Why choose that moment to engage in a particularly damaging harassment campaign?
There are other possibilities, including environmental factors, said experts like Sharon Weinberger, a journalist, and the author of "The Imagineers of War: The Untold Story of DARPA." Others, such as former Foreign Service Officer James Lewis, point to the possibility of human error, in particular a surveillance operation gone wrong.
"Yes, some sort of badly working, nonlethal weapon could be the cause, but a) I doubt it, and b) I don't see the evidence for it yet," Weinberger said. "It's certainly possible, but I think it's too soon to jump to conclusions. You have to look across the board in some organized way what was being reported, what environmental causes could cause that."
"Why would one want to use a weapon that causes inconsistent" harm, she asks.
Lewis likes the odds on his theory.
"There's 100% certainty that American diplomats were under Cuban surveillance," said Lewis, a senior vice president at the Center for Strategic and International Studies. "There's nowhere near that certainty that the Cubans were aiming sonic weapons at American diplomats. It could be harassment, but it's kind of weird."
Lewis says the use sound-based devices to conduct surveillance is common, and that Russia uses the method as well.
Indeed, US investigators are probing whether a third country was involved as "payback" for actions the US has taken elsewhere and to "drive a wedge between the US and Cuba," a US official told CNN.
Lewis outlined the basics. "I bounce a signal off your window and it's affected by sound vibrations, typing on a keyboard or your voice," Lewis explained. "The signal comes back to me and I can use software to read it -- it will say, 'this disruption is caused by the letter Q.'" That allows listeners to piece together what's being said.
Lewis said that if the surveillance equipment was "misconfigured, (it) could produce inaudible noise." His theory: Cuban intelligence agents had set up surveillance on American communications "and it screwed up," Lewis said.
The idea of using sound as a weapon "goes back almost to biblical times," said Weinberger. And indeed, it's used by militaries and police departments, as well as private companies.
The Israeli army has a device known as "the Screamer" that causes nausea and dizziness, according to NPR. American law enforcement officials have used sound cannons to control crowds in the aftermath of Hurricane Katrina in New Orleans and during the G20 meetings in Pittsburgh.
The BBC has reported about cruise ships that use a military-grade sonic weapon to repel Somali pirates. Malls in the UK have used high-frequency sound -- inaudible to most people over the age of 20 -- to discourage teenagers intent on loitering.
But repeatedly, studies have found "these weapons don't work across the board," Weinberger said. "What you see is that sound affects different people differently, and so it's not an effective weapon in that sense."
Whatever injured the US diplomats stationed in Havana clearly had an effect. "We can confirm that at least 16 US government employees, members of our embassy community, have experienced some kind of symptoms," State Department spokeswoman Heather Nauert said Thursday.
The fact that the mystery weapon was inaudible means it was either low frequency sound -- below audible range, also known as infrasound -- or it was high frequency, above audible range, and known as ultrasound.
Dr. Hung Jeffrey Kim, a neurotologist at MedStar Georgetown University Hospital in Washington, said whatever happened in Havana more likely involved low frequency sound because it can travel much farther than ultrasound.
Dr. Scott Masten, a toxicologist at the National Institutes of Health, told CNN that, "At certain amplitude infrasound can cause physiological effects -- it can change heart rate, respiratory rates, blood pressure."
A 2001 survey by the NIH found reports that low frequency sound could cause vertigo, imbalance, "intolerable sensations," incapacitation, disorientation, nausea, vomiting, bowel spasms and "resonances in inner organs, such as the heart."
Low frequency sound can permanently affect balance and hearing, and can travel deep into your brain, causing nerve injury and potentially microhemorrhage, affecting cognitive function and memory if there's long-term exposure, Kim said.
"Most of us know that if you are exposed to a loud noise like a blast, you may experience hearing loss from it," said Kim. "However, if you have the low pitch sound or high pitch sound, like low frequency/high frequency sound, then you cannot really perceive these sounds."
Even though low frequency sound can lead to "permanent damage to your ear, as well as in your brain," Kim said, "you don't know how long you've been hearing it or how loud these noises are."
Both Lewis and Weinberger point to that fact, and say that if the goal was to harass Americans, a silent, invisible attack wouldn't be psychologically effective because the victims wouldn't know they were being harassed.
Lewis said the Cubans have more effective and obvious ways to harass foreign diplomats. "They'll super glue your car -- the key -- that's a popular one," he said. "Or they'll pop your tires. If you're going to harass people, there are a lot more fun ways to do it."