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The parents of James Foley, the journalist whose brutal beheading by Islamic State militants shocked the world last week, have released a letter they say was written by their son in captivity.

The letter was posted late Sunday on the "Free James Foley" Facebook page hours after the conclusion of a memorial Mass in Foley's hometown of Rochester, N.H. A note appended to the posted letter says that Foley's missives were confiscated by his captors with Islamic State, the militant group formerly known as ISIS. The note explains that Foley asked another hostage to memorize one of Foley's letters and relate it to his family after he was released. Foley's parents say that the hostage called Foley's mother, Diane, shortly after his release and dictated her son's letter.

Foley was abducted in northern Syria in November 2012, while covering that country's civil war. On Tuesday, ISIS released a video showing Foley's beheading by a militant with a British accent. Intelligence officials in the U.S. and U.K. have scrambled to identify the militant seen in the video.

The letter begins with Foley reminiscing about past times with his family. "I remember so many great family times that take me away from this prison," he writes. "Dreams of family and friends take me away and happiness fills my heart.

Foley's letter says that he was being held with 17 other hostages. The group passed the time by discussing movies, sports and trivia, while playing games like checkers, chess, and Risk using scraps found in their jail cell. Foley says the hostages were fed daily and given tea and coffee. He also notes "I have regained most of my weight lost last year."

"I have had weak and strong days," Foley continues. "We are so grateful when anyone is freed; but of course, yearn for our own freedom. We try to encourage each other and share strength."

The letter includes messages to different members of Foley's family and concludes with a message to his grandmother.

"Grammy, please take your medicine, take walks and keep dancing," Foley says. ""Stay strong because I am going to need your help to reclaim my life."

Earlier Sunday, in a packed Our Lady of the Holy Rosary church, Roman Catholic Bishop Peter Libasci of the Diocese of Manchester, N.H., said Foley was living his faith by bringing images to the world of people suffering from war and oppressive regimes.

Libasci invoked the prayer of St. Francis, which begins, "Lord, make me an instrument of your peace," to implore the gathered not to hate but to heal.

"It is in giving that we receive," he recited. "It is in pardoning that we are pardoned. It is in dying that we are born to eternal life. To these words, I think we can say, 'Yes, I wish we could do that.' It is not beyond our capability. It is not impossible. Our Lord lived it. Our most Blessed Mother lived it. Many saints have lived it. James lived it."

After the service, Libasci read aloud a letter from the Vatican expressing the condolences of Pope Francis. He also prayed for another captive journalist, Steven Sotloff, and all captives.

"Jim went back again that we might open our eyes," Libasci said. "That we might indeed know how precious is this gift. May almighty God grant peace to James and to all our fragile world."

The Associated Press contributed to this report.