WASHINGTON — Alone in her bed in a sprawling Chinese metropolis, Catherine Werner was jolted awake one night by a pulsing, humming sound. It seemed to be coming from a specific direction.

Perhaps the A.C. unit in her upscale Guangzhou apartment was malfunctioning, the American diplomat thought. But at the same moment, she also noticed intense pressure in her head.

The sounds and sensations returned, night after night, for months. When Werner's health began declining in late 2017 — vomiting, headaches, loss of balance — she brushed it off at first, thinking China's polluted air and water were getting to her.

It wasn't until months later — after her mother, Laura Hughes, grew alarmed, flew in from the U.S. and then got sick, too — that Werner was medevaced from China back to the States. Doctors at the University of Pennsylvania found a vision disorder, a balance disorder and an "organic brain injury" — diagnoses similar to those of 26 U.S. diplomats and spies in Cuba who started hearing strange sounds and falling ill in late 2016.

In May, the State Department issued an ominous health alert: It had "medical confirmation" that a worker in China was affected.

Catherine Werner returned to the U.S. from her diplomatic post in China after reporting symptoms and experiences similar to those of State Department employees evacuated from Cuba. LinkedIn

The United States faced a troubling question: Had whatever happened to the Americans in Cuba spread?

What follows is the first comprehensive account of the extraordinary chain of events set off by a suspected "health attack," as the U.S. calls the mysterious phenomena, on a U.S. worker abroad.

It draws on interviews with more than a dozen current U.S. officials, a written testimonial from Werner's mother to members of Congress obtained by NBC News, internal State Department documents, recorded conversations and other interviews. NBC News also reviewed hundreds of pages of medical records of U.S. government workers evacuated from both Cuba and China, including those the U.S. has "medically confirmed" were attacked and those it ultimately said were not.

For the past 18 months, more than two dozen U.S. diplomatic staffers once stationed in Cuba and China have endured an ordeal that is equal parts medical mystery, political stand-off and bureaucratic muddle.

Cuba and China deny any role. While the U.S. hasn't named a culprit, it says the "health attacks" caused brain injury and other physical harm. Physicians enlisted by the State Department have identified what they call a "Brain Network Disorder" acquired by U.S. personnel serving abroad, say U.S. officials, that includes structural changes to the brain not found in any previously known disorder.

Yet some diplomats and their doctors now tell NBC News they have growing concerns that the U.S. is trying to downplay whatever happened — at least in China.

Equally unsettling to the diplomatic evacuees: suspected incidents of harassment and break-ins they say have occurred since returning to the States. Four U.S. officials tell NBC that the FBI has investigated.

'Something is very wrong'

Laura Hughes awoke one morning at her Pennsylvania farm to find that her husband had booked her a flight to Guangzhou. He'd been watching his daughter Catherine's health decline during regular video chats from halfway around the world and believed the situation had become acute.

"You have to go to China," Hughes recalled him saying. "Something is very wrong."

Hughes arrived in Guangzhou and set about getting Werner new air and water filters and imported food. When mother and daughter noticed signs of home intrusions — lights turned on that had been left off, household items out of place — they adopted two dogs.

Catherine Werner with the dogs she adopted in China. NBC News

Werner, 31, worked with the Commerce Department's Commercial Service and figured the harassment was due to her work on high-profile U.S. trade issues with China. It's not uncommon for U.S. diplomats in hostile countries to suffer harassment or home intrusions intended to remind them they're being watched.

Werner declined to comment for this article. But the facts of her case were confirmed by her attorney, Mark Zaid, who did not object to making Werner's story or elements of her medical records public.

An up-and-comer in the Commercial Service, Werner had grown up partially in China and spoke Mandarin. After studying at Boston's Northeastern University, she had landed jobs at Morgan Stanley and Barclays and worked in Hong Kong before joining the government.

Now she was bumping into furniture, struggling to recall basic words and vomiting for no reason.

"My once beautifully articulate, intelligent and thoughtful daughter had been reduced to a shell of her former self," Hughes would later write to Congress.

Hughes heard the sounds, too — high-pitched and low-pitched — along with pulsing pressure that felt like a wave washing through her body. Soon she, too, developed headaches, nausea and problems concentrating.

She and her daughter became convinced the dogs were being poisoned after they started regurgitating blood, Hughes wrote. On one occasion, they came home to find the dogs' water bowl filled with urine.

Three months after arriving in China, Hughes flew home, feeling she "physically could not tolerate staying any longer." She said she pleaded with Werner to return but couldn't convince her to leave her job.

In late March, Werner reported to the consulate's security office to update the security team on the suspected harassment. But when the officers saw how her health had deteriorated, they referred her to State Department doctors.

A week later, she was given a series of tests called the HABIT, short for the Havana Acquired Brain Injury Tool, which was developed in response to the mysterious illnesses reported by diplomatic staff in Cuba. Twenty-six U.S. staffers had complained of symptoms that included hearing and memory loss and headaches after hearing strange noises and feeling vibrations, usually coming from a specific direction, and always in hotel rooms or their homes. Some Canadian diplomats were affected, too.

State Department medical staff have been trained to administer the triage tests if diplomats report suspicious sounds or symptoms. The exam includes memory tests like recalling strings of numbers and balance tests like standing on one leg, four U.S. officials told NBC News. Those who fail are sent for more advanced testing at the Penn Center for Brain Injury and Repair at the University of Pennsylvania in Philadelphia.

Werner's HABIT tests, given in Guangzhou, showed multiple deficits similar to the Cuba cohort. So a week later, she was sent to Penn, where dozens of doctors performed tests to identify any other possible cause of her problems. Doctors even administered a spinal tap to rule out known nervous system diseases.

A car passes the U.S. embassy in Havana on Oct. 3, 2017. Yamil Lage / AFP - Getty Images file

Among Werner's diagnoses was "neuropsychological dysfunction due to organic brain injury."

Records show she tested below the fifth percentile on a developmental eye movement exam. A psychological evaluation found problems with motor functioning and visual perception, with "clear evidence of decreases in functioning from pre-injury likely." An MRI found "scattered supratentorial white matter lesions," mild but "more than typical for age."

At Penn, a computerized map of Werner's brain was constructed using an experimental MRI technique called diffusion tensor imaging, and experts compared her brain map to a composite created from the cohort of patients affected in Cuba.

Werner's findings were similar enough to the Cuba patients that State Department officials felt they had no choice but to warn the public. So 40 days after Werner arrived in Philadelphia, the U.S. issued its health alert.

"The medical indications are very similar, and entirely consistent with, the medical indications that were taking place to Americans working in Cuba," Secretary of State Mike Pompeo told Congress.

Catherine Werner spent part of her childhood in China and is fluent in Mandarin. NBC News

U.S. medical teams were dispatched to the embassy and five consulates to test any diplomat or family member with concerns. State Department officials said about 300 were tested, including in Guangzhou, Shanghai and Beijing.

After confirming Werner's medical issues, the State Department agreed to pay for her mother to be tested, too — by the same physician treating Werner and the Cuba patients.

Since returning from China, Laura Hughes had been trying to hide her newfound struggle with recalling words from her husband, but he noticed. She grew particularly alarmed when she discovered she couldn't remember the names of the family pets.

An MRI of Hughes' brain in June found chronic small vessel ischemic changes, "more than expected for age." Additional neuroimaging found "scattered foci of abnormal signal in white matter," and an evaluation listed acquired brain injury on her problem list. A neuropsychologist wrote Hughes "continues to experience persistent cognitive impairments, cognitive weaknesses and variable functions after a number of environmental exposures in China."