WEST PALM BEACH (CBSMiami/AP) – A lawyer for an American man imprisoned in Cuba for nearly three years has written to the United Nations’ anti-torture expert saying the treatment of his client “will surely amount to torture” if he continues to be denied certain medical care.

The six-page letter, addressed to U.N. Special Rapporteur on Torture Juan E. Mendez, claims Cuba has not met its obligations under an anti-torture treaty it agreed to more than a decade ago in their treatment of former Maryland resident Alan Gross.

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Gross’ wife, Judy, traveled to West Palm Beach on Sunday for a rally in support of her husband’s release. The rally was scheduled outside a performance of the National Symphony Orchestra of Cuba, which is currently making its first U.S. tour.

Gross, 63, who has lost more than 100 pounds while imprisoned, developed a mass behind his right shoulder earlier this year. Cuban doctors performed tests, but a U.S. doctor who reviewed them for Gross’ family has said they were inadequate. Dr. Alan A. Cohen, a Maryland radiologist, said the mass must be assumed to be cancerous unless proved harmless.

“The lack of medical clarity given to Mr. Gross by the Cubans has been causing him severe mental anxiety for six months and counting,” wrote Gross’ Washington-based lawyer Jared Genser. “As time goes on, and depending on the severity of his illness, the denial of medical care will surely amount to torture.”

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Genser attached to his letter answers to a U.N. form questionnaire for people alleging torture. In it, he wrote the torture occurred from May 2012 to the present — the time that Gross has had the mass on his shoulder.

A senior Cuban diplomat, Foreign Ministry official Josefina Vidal, said in September that Gross’ health “continues to be normal and he exercises regularly.”

Gross has been imprisoned in Cuba since late 2009. He was working as a subcontractor on a USAID-funded democracy building program when he was arrested, and his case has become a source of tension in U.S.-Cuba relations.

He is now serving a 15-year prison sentence. Gross says he was only trying to provide Internet service to Cuba’s small Jewish community. Cuba says the multimillion-dollar programs are an effort by Washington to undermine the government, and has noted that Gross was carrying sophisticated communications equipment.

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Also on Sunday, Gross’ lawyer made public a letter addressed to Cuban President Raul Castro and signed by more than 500 rabbis. The letter urges Castro to release Gross, who is Jewish, on humanitarian grounds.