Gross' daughter and elderly mother both have cancer. State Department officials and his family have expressed hope that Cuba might release him on humanitarian grounds.
"We again call on the Cuban government to immediately and unconditionally release him," said Gloria Berbena, a spokeswoman for the U.S. Interests Section in Havana, which Washington maintains here instead of an embassy. "We will continue to use all diplomatic channels to press for his release. He should be reunited with his family, and bring an end to his ordeal."
Gross, 61, of Montgomery County, Md., was working on a USAID-funded democracy-building program when he was arrested in December 2009. On March 11, he was sentenced to 15 years after being convicted of illegally importing communications equipment.
Cuba considers such programs to be aimed at undermining the government, and he was convicted under a statute outlawing "acts against the independence or territorial integrity of the state."
"Considerable evidence from witnesses, experts and documentation demonstrated his direct participation in a subversive project of the U.S. government to try to destroy the revolution," Thursday's official note read.
Gross has said he was working to improve Internet communications for Cuba's Jewish community. Jewish leaders denied dealing with him.
The case has been a sticking point for relations that have largely been on ice since shortly after the 1959 Cuban Revolution, with Cuba calling Gross a spy and the United States saying no thaw is possible while he remains behind bars.
