Today marks the fourth year that Alan Gross, a 64-year-old former USAID subcontractor, has been imprisoned in Cuba for his USAID work, deemed illegal by the Cuban government.

As The Washington Post reported this morning, Gross has sent a letter to President Obama, through the U.S. diplomatic mission in Havana, detailing the poor conditions of his detention, expressing fears the U.S. government has abandoned him, and asking President Obama for help.

Read more: White House Calls for Merrill Newman's Freedom

One paragraph reads:

As I reflect on these last four years, I find myself asking the same question - why? Why am I still here? With the utmost respect, Mr. President, I fear that my government - the very government I was serving when I began this nightmare - has abandoned me. Officials in your administration have expressed sympathy and called for my unconditional release, and I very much appreciate that. But it has not brought me home.

Gross goes on to plead for help for other Americans held prisoner abroad - a topic that's been in the news lately, as the U.S. seeks the release of Merrill Newman and Kenneth Bae from North Korea, and Robert Levinson from Iran.

Read more: North Korea Says American Apologizes for 'Hostile Acts'

At a press conference in Brussels today, Secretary of State John Kerry was asked about Gross's letter - and about other Americans held overseas. The U.S. has persistently sought their release through back-channels, Kerry said.

"These things are often best resolved in quiet diplomacy, under the radar screen, behind the scenes, and that is exactly what we have been pursuing," Kerry said. "And when, in fact, we secure their release, the track record of those outreaches and those initiatives will speak for themselves as to how much effort and energy has been put into trying to secure their release."

During today's news conference, Kerry also spoke of Alan Gross.

KERRY: In the case of Mr. Gross, we've had any number of initiatives and outreaches over the last several years and engagement with a number of different individuals who have traveled to Cuba, met with people individually there and elsewhere. And we are currently engaged in some discussions regarding that, which I'm not at liberty to go into in any kind of detail.

But the bottom line is we have raised these issues not just in Korea - North Korea - not just in Cuba, but also with respect to a number of Americans who are held in Iran. And I have personally raised those names and those individuals with my counterpart as well as in other ways. And we are hopeful that in each case, at some point we will be able to win their freedom and have them rejoined with their families.

One day is too long to be in captivity, and one day for any American citizen is more than any American wants to see somebody endure. This has been too long in every case, and we will do everything we can and continue to. But these things are often best resolved in quiet diplomacy, under the radar screen, behind the scenes, and that is exactly what we have been pursuing.