SEOUL, South Korea (AP) — In the past year, Choi Jin-young has done everything he can to bring attention to his father, a missionary who’s serving a life sentence in North Korea. He’s met senior U.N., U.S. and other human rights officials, testified at public events, and accepted many media interviews.

But as he marks 10 years since his father’s arrest this week, Choi doesn’t know where his father is, or even if he’s still alive.

“I have some good memories of my dad. I’m his son so I must fight hard to get him returned,” Choi, a 34-year-old beer company official in South Korea, told The Associated Press. “I can only assume he’s been in an extremely bad situation.”

His father Choi Chun-kil, 65, is one of at least six South Koreans who have all but disappeared since being arrested and convicted in North Korea in the past decade or so. Three, including Choi, are Christian missionaries who spent time in Chinese border towns, and the other three are North Korean-born defectors who resettled in the South.

Arrested in unclear circumstances

The three missionaries were involved in covert efforts to spread Christianity in the North by providing North Korean visitors room and board and teaching them the Bible. All three were arrested in 2013 or 2014, and then convicted of plotting to overthrow the North Korean government and spying for South Korea and sentenced to hard labor for life.

Little is known about the defectors, except that North Korea accused one of trying to kidnap North Korean children. Seoul has not released their names, citing requests from family members. But observers say they have little hope of their repatriations to the South as Pyongyang likely views them as its own citizens.

It’s less clear why the North has held the three South Korean-born missionaries for such a long time. North Korea has released or deported American missionaries and other foreign detainees after a maximum of two to three years in prison.

North Korea maintains a virtual ban on religion and considers foreign evangelism a threat to its ruling Kim family’s cult of personality. Experts say North Korea likely lured the South Korean missionaries onto its territory to make the arrests, and fabricated or exaggerated their crimes.

The three missionaries have been denied consular services and opportunities to communicate with their loved ones in South Korea.

They haven’t been seen since the initial months of their detention, when they were shown to the outside world at press conferences in Pyongyang to confess to crimes and apologize. Experts say their statements were likely coached or coerced by North Korean officials.

Conditions could be harsh, but better than those facing North Korean prisoners

Kim Jeong-sam, brother of another jailed missionary Kim Jung Wook, said he prays for his 60-year-old sibling three times a day and misses him whenever he uses hot water.

“Especially when I take shower with hot water on a cold winter’s day, I miss him more because I wonder if he can also use hot water there,” Kim, 63, said.

It’s not known where in North Korea the South Koreans are held. Experts say they’re likely kept apart from North Korean inmates to prevent them from spreading hostile thoughts.

In responses to questions from the AP, South Korea’s Unification Ministry said North Korea hasn’t responded its repeated public calls to confirm whether the detainees are still alive. But it said the North has likely not mistreated the prisoners since South Korea’s appeals to the international community have likely been conveyed to it.

Hyeon Soo Lim, a Canadian pastor who was jailed in North Korea from 2015-2017, believes his foreigners-only prison was in Phyongsong city, about an hour’s drive from Pyongyang, the capital. He said he was mostly the only inmate, except his final months in prison when he encountered American detainee Kim Tong Chol. Lim said he didn’t see the three South Korean missionaries at his prison.

Lim, 69, said he was required to do eight hours of work like harvesting potatoes or digging six days a week. But Lim said he was given a Bible and allowed to exchange letters and a phone call with his family in Canada. He said he was sent to hospital three times.

Kenneth Bae, a former American detainee, described similar conditions in his memoir. Like Lim, he said he didn’t suffer beatings or torture.

But in one exceptional case, American university student Otto Warmbier died in 2017 days after he was released by North Korea in a coma after 17 months in captivity.

Experts say the South Korean missionaries likely won’t have received the same treatment, but their conditions would still be better than those of North Korean prisoners.

Gwon Hyojin, a defector who spent several years at a North Korean prison in the 2000s, said that five to six inmates died every day as a result of malnutrition, beatings by fellow inmates and accidents at work. Choi Min-kyung, who was imprisoned in North Korea before her 2012 defection, said that dozens of people shared a cell and that outbreaks of infectious diseases were also a major cause of deaths.

Can Seoul do more to get its citizens back?

Some experts say North Korea has returned American captives while holding onto South Koreans because it cares more about relations with the U.S. and is deeply hostile to its neighbor.

But others argue that Seoul hasn’t tried hard enough to secure the return of its nationals, due to a lack of public pressure and prioritizing other issues like the North Korean nuclear program or inter-Korean reconciliation.

“I think our media and public haven’t showed much interest in them, and our governments and parliaments haven’t dealt with this issue enthusiastically either,” said Ethan Hee-Seok Shin, a legal analyst at the Transitional Justice Working Group, a Seoul-based human rights NGO.

Shin said attention to the detainees could lead North Korea to view them as valuable bargaining chips, incentivizing it to keep them alive and improve their treatment.

In a departure from Seoul’s longtime stance, the conservative government of President Yoon Suk Yeol has openly criticized North Korean human rights abuses at international events, raised the issue of detainees more and given more support to their relatives. But that effort could be derailed as Yoon faces removal from office after being impeached over his short-lived imposition of martial law earlier this month.

All communication channels between the Koreas have been dormant for several years, with North Korean leader Kim Jong Un branding South Korea a hostile foreign power last year.

Lee Kyu-Chang, a senior research fellow at Seoul’s Korea Institute for National Unification, said that would make it more difficult to secure the release of the missionaries. But Peter Jung, a pastor who specializes in evangelizing North Koreans, said that South Korea shouldn’t give up hope for talks.

“Only when we try to maintain momentum for talks and raise our voices for the detainees, we’ll have a chance to get them back when we sit down for critical talks with North Korea one day,” Jung said.