CHARLESTON, S.C. >> Chris Allen’s phone started buzzing as word broke that invisible attacks in Cuba had hit a U.S. government worker at Havana’s Hotel Capri. Allen’s friends and family had heard an eerily similar story from him before.

The tourist from South Carolina had cut short his trip to Cuba two years earlier after numbness spread through all four of his limbs within minutes of climbing into bed at the same hotel where American government workers were later targeted. Those weren’t the only parallels. Convinced the incidents must be related, Allen joined a growing list of private U.S. citizens asking the same alarming but unanswerable question: Were we victims, too?

It may be that Allen’s unexplained illness, which lingered for months and bewildered a half-dozen neurologists in the United States, bears no connection to whatever has harmed at least 22 American diplomats, intelligence agents and their spouses over the last year. But for Cuba and the U.S., it matters all the same.

It is cases like Allen’s that illustrate the essential paradox of Havana’s mystery: If you can’t say what the attacks are, how can you say what they’re not?

With no answers about the weapon, culprit or motive, the U.S. and Cuba have been unable to prevent the attacks from becoming a runaway crisis. As the United States warns its citizens to stay away from Cuba, there are signs that spring breakers, adventure-seekers and retirees already are reconsidering trips to the island. After years of cautious progress, U.S.-Cuban relations are now at risk of collapsing entirely.

That delicate rapprochement hadn’t even started to take hold in April 2014 when Allen felt numbness overtake his body on his first night in the Havana hotel.

“It was so noticeable and it happened so quickly that it was all I could focus on and it really, really frightened me,” said Allen, a 37-year-old who works in finance.

The Associated Press reviewed more than 30 pages of Allen’s medical records, lab results, travel agency records and contemporaneous emails, some sent from Havana. They tell the story of an American tourist who fell ill under baffling circumstances in the Cuban capital, left abruptly, then spent months and thousands of dollars undergoing medical tests as his symptoms continued to recur.

One troubling fact is true for tourists and embassy workers alike: There’s no test to definitively say who was attacked with a mysterious, unseen weapon and whose symptoms might be entirely unrelated. The United States hasn’t disclosed what criteria prove its assertion that 22 embassy workers and their spouses are “medically confirmed” victims.

So it’s no surprise that even the U.S. government has struggled to sort through confusing signs of possible attacks, odd symptoms, and incidents that could easily be interpreted as coincidences.

The AP has learned that an FBI agent sent down to Cuba this year was alarmed enough by an unexplained sound in his hotel that he sought medical testing to see whether he was the latest victim of what some U.S. officials suspect are “sonic attacks.” Whether the FBI agent was really affected is disputed.

But there’s no dispute that a U.S. government doctor was hit in Havana, half a dozen U.S. officials said.

Dispatched to the island earlier this year to test and treat Americans at the embassy, the physician became the latest victim himself. How badly he was hurt varies from telling to telling. All officials spoke on condition of anonymity because they weren’t authorized to discuss the sensitive investigation. The FBI and the State Department declined to comment.

While the U.S. hasn’t blamed anyone for perpetrating the attacks, President Donald Trump said this week he holds Cuba “responsible .”

Cuba’s government, which declined to comment for this story, vehemently denies involvement or knowledge of the attacks. Miguel Diaz-Canel, Cuba’s first vice president and presumably its next leader, last week called the allegations “bizarre nonsense without the slightest evidence, with the perverse intention of discrediting Cuba’s impeccable behavior.”

When Allen visited Havana three years ago, the sicknesses and political drama were all still in the distant future.

After spending his first day walking the city, he checked into room 1414 of the recently refurbished Hotel Capri. Within minutes of going to bed, he started losing feeling.

The tingling originated in his toes, like that prickly feeling when your foot falls asleep. It spread into his ankles and calves, then to his fingertips. He got up to investigate, and the sensation went away. He got back in bed. The tingling returned, reaching his hands, forearms, ears, cheek and neck.

Allen assumed he’d never identify the cause of all his trouble. Then in September, the AP revealed the hotel where he stayed was the site of other puzzling events — later declared “attacks” by U.S. officials — that left embassy staffers with their own set of varying and seemingly inscrutable symptoms.

“I wanted to wave a flag and be like, I know this, I know what it is like to stay there and have something weird happen to your body and not be able to explain it,” Allen said in an hour-long interview in his office in Charleston.

While the State Department says it’s not aware of any tourists being attacked, it has given credence to the notion that the unidentifiable danger could potentially ensnare any American who sets foot on the island. Its extraordinary warnings last month noted that assaults have occurred at popular tourist hotels, including the Capri, and that the U.S. is no position to guarantee anyone’s safety.

Among the hundreds of thousands of Americans who’ve thronged to Cuba in recent years, Allen isn’t the only tourist who believes he was attacked.

The State Department has received reports of several citizens who visited Cuba and say they’ve developed symptoms similar to what embassy victims experienced. The government says it can’t verify their accounts, but hasn’t indicated it’s trying hard to do so. Asked if anyone is investigating such reports, the State Department said its advice to concerned tourists is to “consult a medical professional.”

Since the AP began reporting on the Cuba attacks, roughly three dozen American citizens have contacted the news agency to say they believe they may have been affected by the same or related phenomena. The AP has not published those accounts, because closer examination gave ample reason to doubt their situations were connected.

Allen’s case is different.

He stayed on the 14th floor of the same Havana hotel where U.S. government workers have been attacked, including on an upper floor. He described sudden-onset symptoms that began in his hotel bed, but disappeared in other parts of the room — similar to accounts given by U.S. government workers who described attacks narrowly confined to just parts of rooms. They also spoke of being hit at night, in bed.

And medical records show Allen conveyed consistent, detailed descriptions of what he experienced to at least six physicians — almost two years before the public knew anything about the attacks.

Still, other parts of Allen’s story don’t neatly align with what embassy workers have reported.

The U.S. has said the attacks started in 2016, two years after Allen’s Cuba visit.

His primary complaints of numbness and tingling aren’t known to have been reported by the government victims, though their symptoms , too, have varied widely and included many neurological problems.

Allen also didn’t recount hearing the blaring, agonizing sound — a recording of which the AP published last week — that led investigators to suspect a sonic weapon. Then again, neither did many of the 22 “medically confirmed” government victims.

When Allen traveled to Havana for a long weekend of sightseeing, Americans were still prohibited from visiting under U.S. travel restrictions that were later eased. He booked flights through Mexico using a Canadian travel company that specifically recommended he stay at Capri, travel records show.

Whatever happened on his first night in Havana, it came back the next evening. Again the numbness set in within minutes of getting into bed, this time stronger and in more parts of his body. It didn’t go away.

So the next morning Allen rushed to the airport and took the first available flight off the island.

But the numbness stayed with him to varying degrees for six months. In that time, he saw an urgent care doctor, then his family physician, and then one neurologist after another at the Medical University of South Carolina.

Every time the numbness seemed to ease, it would return without explanation. Specialists performed nerve conduction tests, full blood workups, exams to check muscle function, a CT of the head, an MRI of the spine, a sonogram of the heart. Doctors considered infections, tumors, the temporarily paralyzing Guillain-Barre syndrome, poisoning from heavy metal contamination and even ciguatoxin, contained in some Caribbean fish.

“When you have these vague symptoms, sometimes all you can do is prove what it’s not,” said Dr. George Durst, Allen’s longtime physician. “No one’s smart enough to figure out what it was.”

Durst said Allen was right to be worried and didn’t imagine his symptoms. He said Allen’s loss of sensation on both sides of the body ruled out peripheral nerve damage, suggesting the problem was in his central nervous system instead.

Outside medical experts say it’s difficult if not impossible to determine whether different symptoms experienced by different individuals in Cuba are causally connected. The U.S. has declined to say what criteria separate the medically confirmed victims from others who’ve reported concerns or symptoms.

“I am sure that between April 2014 and October 2017 there must have been a very large number of people who were in Cuba and who were affected by various symptoms. But that’s not unusual,” said Mario Svirsky, who teaches neuroscience at New York University School of Medicine.

If Allen was targeted by anyone, it’s not clear why.

He would have been one of the first Americans to come through Hotel Capri after a major renovation. The iconic high-rise, known as a flashy mobster hangout before Cuba’s 1959 revolution, had re-opened a few months earlier under a partnership between Cuba’s state-run tourism company and a Spanish hotel chain. Hotel spokespeople declined to comment for this story.

To an outsider, Allen could have looked like a U.S. government agent, potentially even a spy.

A clean-cut 33-year-old at the time, he had worked for years in Republican politics, including on former Utah Gov. Jon Huntsman’s 2012 presidential campaign. He also performed “advance” work in George W. Bush’s administration that involved setting up logistics for official trips, a contract job that meant he briefly had an official passport.

Allen approached the AP after it reported on the Capri attacks to ask how he could contact investigators to volunteer information. He agreed to tell his story publicly once it became clear the U.S. government was not actively looking into cases of potentially affected tourists. Allen said he was uninterested in publicity, and declined AP’s requests to be photographed and to tell his story on camera.

The harrowing symptoms aside, Allen said he doesn’t regret visiting Cuba. Eight months after his trip, as former President Barack Obama and Cuban President Raul Castro announced they would restore diplomatic relations, Allen took to Instagram to reflect on “a few wonderful days wandering the streets and photographing the people of Havana.”

“If the latest news makes it easier for you to visit, I encourage all of you to do so sooner than later,” he wrote.

AP Medical Writer Lauran Neergaard and news researcher Monika Mathur contributed to this report along with Matthew Lee and Bradley Klapper in Washington and Jake Pearson in New York.