For William Cox, this will be the first Christmas he celebrates without his mother Leyla after she died under mysterious circumstances during a June vacation to the Dominican Republic. He attempted to have a blood sample shipped to a U.S. laboratory to hopefully uncover answers, but it was lost by FedEx, which said it had done everything in its power to locate it.

Cox, 25, was told his mother Leyla, 53, died of apparent heart failure in her hotel room in Punta Cana. Following her passing, Public Health Minister Rafael Sanchez Cardenas claimed she had an enlarged heart, high blood pressure and had suffered “several past heart attacks.”

Cox disputed these claims in various media appearances, as did his mother's supervisor from Richmond University Medical Center in New York, where she worked.

Cox even had Dr. Mehmet Oz examine his mother's records during an episode of "The Dr. Oz Show," and Oz agreed she had no heart condition. He also said the show's staff was stonewalled during their investigation and suggested something nefarious may be going on behind the scenes.

"You know, when smart people don't get things done, it's not by accident," Oz told Cox. "Sometimes there's a reason why you're not finding out stuff you're supposed to find out."

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Lelya's body was eventually cremated in the Dominican Republic, leaving more questions than answers.

After working with the U.S. Embassy, Cox obtained a blood sample prior to cremation. It was shipped to the Richmond University Medical Center to be studied by Medical Examiner Dennis Cavalli, as a courtesy.

Before the medical examiner could examine it, Cox claims the FBI intervened and told Cavalli to "stand down."

Cavalli referred Fox News to Aja Worthy-Davis when reached for comment. Davis, who works in the Office of Public Affairs for New York's Chief Medical Examiner (OCME), told Fox News in a statement: "OCME does not have jurisdiction over deaths that occur outside of NYC. As a courtesy, as the decedent was a NYC resident, we offered assistance. We have not been involved any further."

Cox then tried to ship the blood sample to a lab in Texas and was told it had been lost at FedEx's facility in Memphis, Tenn. FedEx sent him a rebate check for $100 and considered the matter closed. Cox persisted and claimed the shipping giant asked him for more time to find the parcel and to investigate the matter fully. Two months later, still no word.

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“FedEx appreciates the sensitive nature of this shipment, and we offer our sincere regret and condolences to Mr. Cox and his family. After an exhaustive search, we were unable to locate the shipment and the claims process has been completed," the company said in an email.

Cox expressed skepticism and said FedEx's response wasn't good enough.

"I don't see how FedEx can sit here and say, 'We regret to inform you that this package that you've been working for since June just went missing in our facility and we offer our condolences.' I mean, that package cost me 15 grand," he told Fox News. "I just want to know how my mother passed away and how she died. Honestly, I don't know what to do."

He added, "You don't lose a package in 2020. There is a track record for everything. I find it hard to believe it simply vanished. If they misplaced the package or destroyed it, they need to be held responsible ... Things just don't disappear."

Fox News also reached out to the FBI about Cox's claim that they shut down the blood test, but they declined to weigh in and referred Fox to the State Department.

"Whenever a U.S. citizen passes away overseas, the Department of State works to provide all appropriate consular assistance to their family," a State Department official told Fox News. "With regard to this case, as in all such cases, the Department followed the directives of the family of the deceased and provided them with the findings of local authorities."

"We would refer questions about private testing to Ms. Cox’s family or the laboratory responsible for that testing," the official added.

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Cox said every time he spoke with the State Department about his mother's death, they would refer him back to the FBI.

He had previously taken it upon himself to reach out to the FBI after he was told they halted Cavalli's examination. He claimed they hung up on him twice before finally telling him they couldn't be of assistance.

"They listened to what I had to say and told me, 'We know who you are, Mr.Cox, and we are sorry to inform you we can not release any information regarding whether or not the death of Leyla Cox was under investigation,'" he said. "I called Dennis Cavalli back, to see who exactly told him this information. He couldn’t tell me and said, 'I want to help you, Mr. Cox, but my hands are tied since the FBI stepped in.'"

He remains baffled and pained, as he begins to question the system that has kept him running in circles since day one.

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"I have been dealing with misdirection and misinformation since the first phone call with the U.S. Embassy," Cox said.

He added, "The FBI has not been helpful. If anything they prevented a toxicology test from being performed in New York. I don’t know why they would do something like that, I don’t know why I've been misled at every turn."