NEW YORK — Mr Kenneth Bae says his North Korean captors interrogated him up to 15 hours a day for the first four weeks of his incarceration and yelled with impatience until he wrote a confession to their liking. But they would later allow Mr Bae to read his Bible and permit him to pray.

They told Mr Bae that nobody from America cared enough to negotiate his freedom and that he would most likely spend his entire 15-year sentence of hard labour in North Korea’s penal system, where he toiled on a soybean farm and lost more than 13.6kg. But they let Mr Bae read hundreds of letters emailed from the United States by his family and friends.

The North Koreans also billed Bae about US$300,000 (S$401,831) for hospital expenses as one of the conditions for leaving, but apparently dropped that demand in the final phase of his ordeal, which ended abruptly after two years in November 2014. The bill, he said, was never paid.

Mr Bae, 47, a missionary and a father of three from Washington state who was the longest-held US prisoner of North Korea, revealed these previously unreported details about his imprisonment in a book, Not Forgotten, which goes on sale Tuesday (May 3).

He was freed with another American, Mr Matthew Todd Miller, in what the North Korean authorities described as a magnanimous gesture by their leader, Mr Kim Jong Un. They were flown home on a US jet by Mr James R Clapper Jr, the Obama administration’s director of national intelligence, who had visited Pyongyang to secure their release.

Mr Bae’s book, a chronology of events as he saw them, is written as a religious tale, sprinkled with scripture he inserted to reinforce his theme of hope versus despair and his belief that the Christian God had protected him in a country where sharing such thoughts is considered subversive. But the book is also a survival story that shines a light on North Korea’s history of seizing US citizens.

About a dozen Americans have been arrested in North Korea in recent years. All but two have been released, usually through intervention by top US officials. Mr Otto F Warmbier, a college student from Ohio, was sentenced in March to 15 years of hard labour for trying to steal a political banner. Mr Kim Dong-chul, a businessman from Virginia, was sentenced on Friday to 10 years of hard labour for spying and other offences.

In an interview before the book was released, Mr Bae said he knew by the third night of his detention, in November 2012, that he should start keeping a journal. “I felt like I had a story to tell, so I decided that maybe it was something I could write about,” he said.

A naturalised American originally from South Korea, Mr Bae was arrested in the North Korean city of Rason while leading an officially permitted tour from China, where two years earlier he had started a travel business that specialised in North Korea.

His underlying motive in that business was missionary work, which is illegal in North Korea. The North Korean authorities discovered his real purpose by examining files in a computer hard drive that Mr Bae says he had inadvertently left in his luggage.

The first month of captivity, he said, was among the most difficult, as he struggled to explain why he had such material.

“I felt like an insect, tangled in the spider web,” he said. “Every time I moved it got messier, with no way out.”

Mr Bae said he was questioned “from 8 in the morning until 10 or 11 o’clock at night, every day for four weeks. It was very intense.”

His treatment improved after he wrote a confession that satisfied his interrogators, in which he described himself as a terrorist who had plotted to overthrow the government. Mr Bae said his chief prosecutor told him he was “the most dangerous American criminal apprehended in the 60 years since the Korean War”.

While his prison cell was spartan, Mr Bae said he was far better off than North Korean felons. The penal camp where he was assigned was apparently meant only for foreigners, even though he never saw another prisoner.

“It wasn’t the Marriott hotel,” he said in the interview. “It wasn’t even Motel 6.”

During his confinement, Mr Bae was hospitalised three times for problems including diabetes, an enlarged heart and back pain. The North Korean authorities, he said, wanted to charge him €600 (S$927) per night in a hospital, which he said worked out to about US$300,000. “I told them, ‘I’m a missionary; I don’t have that kind of money’,” he said.

While his prison treatment was harsh, Mr Bae said it also revealed a softer undercurrent of respect toward foreign citizens by the North Korean authorities, who are concerned about their image abroad.

He was never beaten, Mr Bae said. He was allowed to communicate with family members — his mother was even permitted to see him in the hospital. He still carries the Bible he was allowed to keep during his imprisonment.

“What is most surprising is the access Kenneth Bae had to messages from his family,” said Mr Bill Richardson, a former diplomat who has visited North Korea to negotiate the release of imprisoned Americans, and who wrote the foreword to Mr Bae’s book. Nonetheless, Mr Richardson said in a telephone interview, “Kenneth’s ordeal was the toughest.”

As Mr Bae realised he was about to be released, he wrote in the book, a prosecutor whom he had nicknamed Mr Disappointment for repeatedly predicting he would never leave, told Mr Bae he had done so with good intent.

“I did not want you to get your hopes up only to have them crushed,” Mr Bae quoted him as saying.

Mr Bae also wrote that he had known about a trip to North Korea in January 2014 by Mr Dennis Rodman, the former basketball player and a friend of Mr Kim’s, but wrote that he only later learnt that Mr Rodman had disparaged Mr Bae in a CNN interview, rather than calling for his release. Mr Rodman’s remarks, for which he later apologised, incited outrage by Mr Bae’s relatives in the United States and indirectly galvanised publicity over his case.

“I thank Dennis Rodman for being a catalyst for my release,” Mr Bae said in an interview with CNN broadcast Monday. “He brought attention to my plight.” NEW YORK TIMES