COLUMBIA, S.C. (AP) — U.S. Rep. Joe Wilson on Thursday called on the U.S. State Department to investigate a bizarre string of attacks on diplomats in Havana, urging Secretary Rex Tillerson to get to the bottom of unexplained, invisible attacks that have hurt Americans on Cuban soil.

"In light of these serious attacks on U.S. embassy staff, we urge the State Department to immediately investigate the incidents and figure out what is going on in Havana," Wilson, a South Carolina Republican and House Foreign Affairs Committee member, wrote.

Nearly two-dozen American diplomats, intelligence agents and their spouses in Havana have been harmed in the attacks over the last year. The State Department has said 22 Americans are "medically confirmed" to be affected, although the number could grow.

The symptoms and circumstances reported have varied widely, making some hard to tie conclusively to the attacks. The incidents began last year and are considered "ongoing," with an attack reported as recently as late August.

Wilson's letter called on Tillerson to do what the State Department and the U.S. government have already been doing for the better part of a year. The FBI is leading an investigation to determine what and who is attacking U.S. personnel and how to stop it, the State Department has said. That investigation also includes the State Department's Diplomatic Security division, among other agencies, The Associated Press has reported. The FBI has traveled to Havana with advanced equipment and has ripped open walls to try to find any device that could be responsible — the first time the FBI has been allowed to operate openly on Cuban soil in more than half a century.

President Donald Trump has blamed the island nation, even though Cuba's government has repeatedly denied both involvement in and knowledge of the incidents. On Monday, Trump said, "I do believe Cuba's responsible. I do believe that," offering no new details about what type of weapon might have caused damage ranging from permanent hearing loss to mild traumatic brain injury, or concussion.

The State Department has said that despite the lengthy investigation and FBI visits to the island, the U.S. still can't identify either a culprit or a device. It also wasn't clear whether Trump meant Cuba was behind the attacks or merely shared the onus because it failed to keep Americans safe on its soil. In his letter, Wilson wrote the allegations raise concerns about not only the U.S. diplomatic staff but also "about the future health and well-being of American citizens visiting the island."

At least one tourist has reported experiencing an unexplained illness that lingered for months following a 2014 stay in the same hotel where American government workers were later targeted. Chris Allen, from South Carolina, told AP that he experienced numbness that spread through his limbs within minutes of climbing into bed.

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 has said 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."

The State Department has avoided casting blame on Cuban President Raul Castro's government for the attacks that began last year and have eluded an FBI investigation. In a cable sent Monday to all overseas U.S. diplomatic posts, the State Department said it has "not assigned blame to the Government of Cuba."

"We are still investigating these attacks and do not know who or what is behind them. We continue to exchange information with Cuban investigators," said the diplomatic cable. Marked "sensitive," the cable laid out the rationale for the steps the U.S. has taken in response to the attacks, including pulling more than half its diplomats from Cuba and kicking out 15 Cuban diplomats in the United States.

The U.S. and Cuba re-opened diplomatic relations in 2015 after a half-century of estrangement, but the attacks on Americans and steps taken by Washington in response have started to unravel those budding ties.

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Kinnard can be reached at http://twitter.com/MegKinnardAP. Read more of her work at http://bigstory.ap.org/content/meg-kinnard/