South Korean families pray for relatives who vanished into North Korean prisons a decade ago

Choi Jin-young fights to bring attention to his father, a missionary imprisoned in North Korea for a decade.

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.

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.