American prisoners have previously been released after high-profile visits or communications from the US. Even if the individuals are merely pawns in a wider political game, the impact on their lives is great.
Who gets detained and when?
North Korea said last year it would treat US detainees under its "wartime law," after Washington added leader Kim Jong Un's name to the sanctions list for alleged human rights abuses and censorship.
At the time, state media KCNA reported"The Republic will handle all matters arising between us and the United States from now on under our wartime laws, and the matters of Americans detained are no exception to this."
The change was unwelcome news for US citizens already in custody, and served as a warning to others who might follow.
The latest travel warning from the US State Department lists acts that have been treated as crimes in North Korea, whether or not they were done knowingly. They include taking unauthorized photographs, shopping at stores not designated for foreigners and carrying out religious activities.
Of the four Americans currently being held, two were said to have committed "hostile acts" against North Korea. University of Virginia student Otto Warmbier removed a political banner from a hotel; Kim Sang Duk, also known as Tony Kim, was accused of attempting to overthrow the government. Kim Dong Chul was sentenced for being a spy. There is no information yet about possible allegations against Kim Hak-sung, who was detained this month.
By the time a trial is scheduled, guilt is often admitted, which makes the only unknown the sentence that will be handed out. But previous prisoners have said their confessions were not made voluntarily. And when CNN has interviewed detainees, North Korean officials have always been in the room, making it impossible to judge if statements of guilt and regret are being made under duress.
With Otto Warmbier, for example, he admitted in an emotional press conference that he had tried to steal a political banner and blamed US officials. "I never, never should have allowed myself to be lured by the United States administration to commit a crime in this country," he said tearfully as he begged for forgiveness.
N. Korea sentences U.S. student to 15 years hard labor
Later, his trial was said to have lasted only about an hour. In the court, North Korean officials presented fingerprints, photos of a political banner and surveillance images -- proof, they said, that Warmbier committed crimes against the regime.
Where do they serve their sentences?
Warmbier was sentenced to 15 years' hard labor -- which can mean grueling farm work.
Kenneth Bae, who was arrested in November 2012 and later sentenced to 15 years' hard labor for committing unspecified "hostile acts" in the country, said he worked outside. "I worked from 8 a.m. to 6 p.m. at night, working on the field, carrying rock, shoveling coal," Bae said on CNN's "New Day" after his release.
Adding to the physical pain was the verbal abuse he received from North Korean officials, Bae said.
He said one prosecutor repeatedly told 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.'"
Kenneth Bae describes years in North Korean labor camp
"I've been going back and forth between hospital and to the labor camp for the last year and a half," Bae said in 2014.
The then 46-year-old, who suffered from diabetes, high blood pressure and kidney stones, said his health was failing.
"My hands are numb and tingling, and it's difficult sleeping at night, and I was working in the field every day," Bae said. By the time he was released, Bae had lost 60 pounds in weight.
Journalist Laura Ling, arrested with colleague Euna Lee for entering the country illegally, was sentenced to hard labor but never delivered to a prison camp. Still, she told her family of the deprivations, remarking at the time of her release that she was looking forward to eating fresh fruit again, as so many meals over the previous months had been rice, often containing rocks.
Canadian pastor Rev. Hyeon Soo Lim, who is still detained, said he dug holes in an orchard for eight hours a day. As a Canadian, he has had occasional consular visits and some letters from family. American detainees may get visits from Swedish officials on behalf of the US, which does not have diplomatic relations with North Korea.
How do they get out?
In keeping with the view that Western detainees, particularly Americans, are held as part of a bigger political scenario, it is often some grand gesture that precedes a release.
For Ling and Lee, it took a visit by former US President Bill Clinton to then North Korean leader Kim Jong Il to get them freed.