Image copyright Reuters Image caption American student Otto Warmbier has been released - but is in a coma

There is no doubt that North Korea treats its prisoners harshly.

When outsiders are arrested, they are often sentenced to hard labour, and that's exactly what it is - compounded by the severe oppression of isolation and helplessness.

The BBC knows of one former prisoner who was broken psychologically by his treatment. Many years later, he remains too traumatised to talk about it easily.

But others have described their experience in detail.

In December 2012, North Korea charged missionary Kenneth Bae with acts "hostile to the republic".

He had visited the country many times, but was stopped on this occasion and a hard drive with Christian material was discovered.

For this "crime", he was sentenced to 15 years hard labour, and only released when his health deteriorated seriously - just as seems to have happened in the current case of Otto Warmbier.

After his release, Mr Bae wrote a memoir, "Not Forgotten: The True Story of My Imprisonment in North Korea" in which he said that he was interrogated from 08:00 in the morning until 22:00 or 23:00 at night every day for the first four weeks of his imprisonment.

Image copyright Reuters Image caption Kenneth Bae worked six days a week on a farm, "carrying rock, shovelling coal"

Under this pressure, he wrote the hundreds of pages of confessions his interrogators demanded.

Mr Bae said he would work six days a week on a farm, "carrying rock, shovelling coal".

His daily routine was to wake at 06:00, eat breakfast, pray, and then be taken to perform the hard labour from 08:00 until 18:00.

Under this regime, he lost a lot of weight - an estimated 60lbs (27kg) in the 735 days of his captivity.

As his weight dropped, his health increasingly failed and he was repeatedly taken for medical treatment.

Apart from the physical toll, there was a psychological pain, a feeling of isolation.

He said one interrogator kept telling him: "No-one remembers you. You have been forgotten by people, your government. You're not going home anytime soon. You'll be here for 15 years. You'll be 60 before you go home".

He said: "I felt like an insect, tangled in the spider web. Every time I moved it got messier, with no way out."

He does say that, later on, after the month of interrogation was over, he was allowed to see emails and messages from people back home (though this may have been both a comfort and a torment). He seems to have been allowed a bible.

When he became seriously ill, it looks as though the North Korean authorities became concerned that he might die, with all the diplomatic difficulty that would cause.

And so they arranged his release - as it appears might have happened in the case of Otto Warmbier.

Dig their own graves

Kenneth Bae is an American citizen originally from South Korea, and so spoke Korean. He said he thought his treatment as a prisoner with a cell of his own, including a bed and a toilet, was not as tough as that for North Koreans held in the vast array of camps for ordinary crime or for dissent.

He may be right on this. Amnesty International has described the prison camps as harsh beyond endurance.

"Hundreds of thousands of people - including children - are detained in political prison camps and other detention facilities in North Korea," it says.

"Many of those have not committed any crime, but are merely family members of those deemed guilty of serious political crimes".

Amnesty analyses aerial pictures of the camps and says that one of them is three times the size of Washington DC contains 20,000 inmates. According to one former official it had talked to, detainees were forced to dig their own graves and rape was used as punishment, the victims then disappearing.

Image copyright Amnesty Image caption Amnesty tracks developments at some of the known camps in North Korea, which stretch for miles

Kenneth Bae does not say he was physically tortured or beaten. His decline in health was because the harshness of the prison regime exacerbated his diabetes, high blood pressure and a kidney condition.

That may or may not be the case of Otto Warmbier. But there are questions which the authorities in the United States are surely asking: How did he end up in a coma? And why did North Korea take a year to tell any outside country?

If it was because of some sort of physical attack, there might be political pressure on President Donald Trump to get tougher with the regime in Pyongyang.

There is one other intriguing question: do prisoners influence their jailers?

Stockholm Syndrome is when a hostage comes to identify with the people holding him or her, but is there a reverse Stockholm Syndrome?

Kenneth Bae found that his captors were curious about life in the West. They wanted to know how much a house cost, and whether many people really did own their own home and car.

"At the beginning, it was difficult but because I speak the language I was able to communicate," he said later, after he was freed. North Koreans had been told of a grim life in America where 99% of people lived in poverty.

"I told them most people own a house and a car, and they said: 'That can't be right'."