What you need to know about the 3 Americans still detained in North Korea

— -- The death of U.S. student Otto Warmbier has brought a wave of sorrow and a new stumbling block to any sort of negotiations between the United States and North Korea. But as the United States determines how to retaliate, if at all, there is another enormous risk to consider: the three U.S. citizens still detained by the country's authoritarian regime.

North Korea revealed on May 7 that Kim Hak-song had been detained on charges such as espionage and crimes against the state, making him the third American now being held and the 10th in total to be detained by North Korea's current leader, Kim Jong Un. Only one country has detained more Americans than North Korea: Iran, where five Americans are missing or detained.

Although Donald Trump tweeted during the campaign that similar detentions would not happen if he became president, Kim Hak-song is also the second American to be detained since Trump's inauguration.

As the Trump administration works to secure their release and thwart the North's nuclear and missile program, it will have to weigh pushing the regime too far on bringing these three men home. Here is what we know about the U.S. citizens detained in North Korea.

Kim Dong-chul

The American who has been held the longest in North Korea at this time is Kim Dong-chul. A naturalized U.S. citizen who was born in South Korea, he is a businessman in his mid-60s who at one time lived in Fairfax, Virginia.

He was living in China near the border with North Korea and working in a special economic zone in North Korea as the president of a trade and hospitality company. After years of openly crossing the border, he was detained in October 2015, accused of being a spy for South Korea and sentenced to 10 years of hard labor.

He has a wife and two daughters in China. His detention wasn't publicly known until January 2016, when a CNN crew was allowed to interview him.

Tony Kim

Tony Kim, who also goes by his Korean name, Kim Sang-duk, is a 58-year-old U.S. citizen who was temporarily teaching an accounting course at the Pyongyang University of Science and Technology, North Korea’s only private university.

He was detained at the airport while trying to fly to China with his wife on April 22. It is unclear what prompted his detention, but he has been charged with hostile criminal acts with an aim to subvert the country.

He was the first American detained under Trump.

Kim Hak-song

Little is known about Kim Hak-song.

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He was being held for "hostile acts against the country," the regime's official news agency said, although details about the alleged acts were not provided by North Korean authorities.

Kim had also been working at the Pyongyang University of Science and Technology, in agricultural development with its "experimental farm," the university said in a statement. He was arrested May 7 as he was "about to leave … after a visit of several weeks," it added.

The university said that Kim Hak-song, like Tony Kim, was detained not because of his university work but for other unspecified reasons.

Other Americans Next?

The number of Americans in North Korea at any given time fluctuates and is difficult to determine. But one of them could easily become the next detainee in a country where a brutal regime monitors its denizens' every move.

As of last month, there were around 40 Americans, the majority of whom are ethnic Koreans, on the faculty of the Pyongyang University of Science & Technology, where both Tony Kim and Kim Hak-song worked.

In addition, between 800 and 1,250 Americans visited North Korea annually in past years, according to tour companies working in the reclusive country. Those numbers may decline after Warmbier's death, especially because Young Pioneer Tours -- the group that organized Warmbier's tour -- has canceled future trips for Americans.

Warmbier, who was imprisoned in North Korea for more than 17 months and was returned home to Ohio last week in a coma, died this week, his family said Monday.

The State Department strongly recommends Americans not travel to North Korea.