Otto Warmbier, the American college student detained in North Korea for more than 17 months, was returned Tuesday to his hometown of Cincinnati. His release brings the number of U.S. citizens known to be in detention in the hermetic communist dictatorship down to three.

Warmbier was arrested in Pyongyang in January 2016, while visiting the country on a group tour. The then-21-year-old student was accused of trying to steal propaganda signage from a restricted area in his hotel and convicted of committing “hostile acts” toward the regime of Kim Jong Un. He was sentenced to 15 years hard labor, and he had not been seen or heard from since he appeared in a dramatic confession apologizing for “the biggest mistake” of his life. In a statement confirming Warmbier’s homecoming, his parents said they were told their son had been comatose since March 2016, the month he was convicted. A North Korean official told a U.S. counterpart that Warmbier had contracted botulism and fell into the coma after taking a sleeping pill, CNN reports. Warmbier’s parents said they learned of their son’s condition “only one week ago.”

Read More: How Otto Warmbier Made It Out Of North Korea

The uncertainty over Warmbier’s condition and treatment in detention speaks to the opacity of North Korea’s justice system. But accounts of former prisoners have shed some light on the harsh conditions he and other detainees might have endured. In his memoir Not Forgotten, missionary Kenneth Bae, who was imprisoned for more than two years, wrote of daily interrogations “from 8 in the morning until 10 or 11 o’clock at night, every day for four weeks.” Another detainee, journalist Laura Ling, has spoken of being imprisoned in a 5-by-6-foot cell, the New York Times reports. Three other U.S. citizens are believed to still be in North Korean custody. All are Korean-American, and two were in the strictly secular country working with a Christian-backed university based in China, near the Korean border. The third was a businessman.

Less is known about these three prisoners than Warmbier, whose friends and family have helped shed some light on his strange and unfortunate case. Here’s what we know about the three Americans who are believed to remain in North Korean custody.

1. Kim Hak Song

Authorities are believed to have arrested Kim Hak Song, who had been in North Korea for several weeks working at the Pyongyang University of Science and Technology (PUST), on May 6 this year as he was about to leave the country. The Korean Central News Agency (KCNA) reported his arrest for “hostile acts” toward the state the following day, but offered no details about his age, occupation or alleged crime.

The university said in a statement that Kim, also known by the Chinese version of his name Jin Xue Song, was apprehended after concluding a trip to carry out “agricultural development work” at the school’s “experimental farm.” Kim is the second American staffer at the school to be arrested this year after his colleague, Tony Kim, was apprehended in April. It is unclear what, if any, connection the two prisoners may have had with each other.

Read More: Kim Jong Un Effectively Made Dennis Rodman a U.S. Ambassador. That’s The Best Reason Yet to Reengage

North Korea’s first private university, PUST was founded and remains operated by actors outside the country, mostly Evangelical Christian organizations. While it is a secular school — North Korea keeps a tight grip on information about religion — the university chiefly employs Christian staff. The school has close historical ties to Yanbian University in Jilin, China, near the border with North Korea. Tony Kim reportedly worked as a professor there, while Kim Hak Song is believed to have studied and later worked at the school. Representatives of PUST have stated that the arrests of Tony Kim and Kim Hak Song are “not connected in any way” with the university’s work.

Born in Jilin and educated mostly in California, Kim was an ethnic Korean who is believed to have emigrated to the U.S. in the mid-1990s, according to CNN. Citing two men who said they studied with him in the U.S., CNN reports that Kim likely became a citizen in the 2000s, then later moved back to China. His classmates described him as a proud Korean with a passion for issues related to food security and agriculture. “North Korea is persecuting their savior, a person who came to help them. This is wrong,” David Lee, one of the men who said he studied with Kim, told CNN.

2. Tony Kim

Tony Kim, also known by his Korean name Kim Sang Duk, was also a teacher at PUST. Few further details are known about the man, believed to be in his 50s, who was abducted at the airport in Pyongyang while trying to leave the country on April 22. Reports in Korean state media did not specify the allegations against him. Upon his arrest, PUST issued a statement that his detention was “related to an investigation into matters not connected in any way with the work of PUST,” and that “life on campus and the teaching at PUST is continuing as normal.” (An almost identical statement was released in response to his colleague’s arrest.)

The chancellor of the university, Chan-Mo Park, told the New York Times that Kim was involved in some extracurricular activities, such as volunteering at an orphanage. According to the Times, Kim most recently lived in North Korea with his wife, who may still be in the country. Citing the prisoner’s Facebook page, the Times reported that Kim studied accounting at the University of California, Riverside and Aurora University, and spent about a decade working as an accountant in the U.S. before returning to Asia.

3. Kim Dong Chul

South Korea-born Kim Dong Chul, now aged around 63 or 64, was arrested in Oct. 2015 on charges of espionage and other undisclosed crimes. Kim is a naturalized U.S. citizen and resident of Fairfax, Va., who ran a trade and hotel services firm in the special administrative zone between China and the DPRK. His secretive detention was only brought to public attention about three months later, when authorities decided to introduce the prisoner to a CNN crew visiting Pyongyang. He was convicted in March 2016 and sentenced to 10 years hard labor, about a week after Warmbier’s conviction — both sentences were handed down shortly after the U.S. had levied steeper sanctions against North Korea in response to missile testing.

Kim also delivered a public confession and apology, wherein he said that in 2013 he began spying on behalf of “South Korean conservative elements,” the New York Times reported at the time. The North Korean government reportedly arranged a limited press conference that was covered by South Korea’s Yonhap News Agency and China’s state-run Xinhua. The few reports that exist about the conference said he admitted to bribing North Korean residents for information about Pyongyang’s nuclear weapons program.

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