WASHINGTON (AP) — An American man marking four years in a Cuban prison is asking President Barack Obama to get involved in securing his release.

Alan Gross was arrested four years ago Tuesday while working covertly in the Communist country to set up Internet access for the island's small Jewish community. At the time, he was working as a subcontractor for the U.S. government's U.S. Agency for International Development.

Cuba considers USAID's programs illegal attempts by the U.S. to undermine its government, and Gross was ultimately tried and sentenced to 15 years in prison.

Gross wrote in a letter released Tuesday that it is clear to him that only with Obama's "personal involvement" will he be freed.

Gross' case has been a sticking point in improving the U.S.-Cuba relationship.

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