Alan Gross, a Jewish-American contractor who has been imprisoned in Cuba for the last four years, sent a letter to US President Barack Obama on Tuesday, the anniversary of his arrest by Cuban authorities, asking the president to personally intervene to secure his release.

“I fear that my government – the very government I was serving when I began this nightmare – has abandoned me,” Gross wrote in a letter posted online by the Washington Post.

Gross wrote that he has “lost almost everything” during his four-year imprisonment, in which he has ”been confined 23 hours a day to a small cell with two fellow inmates” where the lights are kept on at all times. “With the exception of a few phone calls and visits, I am completely isolated from the outside world,” he added.

Only the “personal involvement” of Obama can secure his release, Gross wrote. The US government has “taken extraordinary steps to obtain the release of other US citizens imprisoned abroad – even citizens who were not arrested for their work on behalf of their country. I ask that you also take action to secure my release, for my sake and for the sake of my family,” he added.

Administration officials “have expressed sympathy and called for my unconditional release, and I very much appreciate that. But it has not brought me home,” Gross said.

On Monday, the US State Department said that it found it “gravely disappointing” that “the Cuban Government has not allowed Mr. Gross to return to his family, where he belongs.” Gross should be released “immediately and unconditionally” from his “unjustified imprisonment,” the department said in a press release.

Gross, then a subcontractor for the State Department on a mission to connect Cuba’s small Jewish community to the Internet, was arrested in December 2009 as he was leaving Cuba. He is serving a 15-year sentence for “crimes against the state.”

Gross’s wife and Washington, DC’s Jewish community called last week on Obama to work to secure his release. A rally was called for Tuesday at Lafayette Park outside the White House.

Judy Gross said last week that her husband, 64, was depressed and in chronic pain from arthritis.

“The best thing to do is contact the White House,” she said she planned to tell people at the rally. “Ask them to do what you need to do to get Alan home.” She would not elaborate except to say that the “president has the power to do what it takes to get him home.”

The Cuban government has indicated that it wants the United States to allow the return to Cuba of five spies currently in prison or on probation in the United States.

JTA contributed to this report.