WASHINGTON, D.C. - The son of a U.S. citizen detained in North Korea is hoping against hope that his father will be released in conjunction with the unexpected summit between President Donald Trump and North Korean leader Kim Jong Un.

"I'm thankful that President Trump is going to have this summit. I'm thankful for his work and what he's doing. I'm hoping the issue of my dad and other detainees would be brought up," said Sol Kim in an interview with Voice of America's Korean Service on Wednesday.

His father, Kim Sang Duk, whose American name is Tony Kim, has been detained in North Korea since April 22, 2017 when he was arrested at Pyongyang International Airport. North Korean state media reported that Kim had been arrested for "committing criminal acts of hostility aimed to overturn" the country and he was held in custody pending a "detailed investigation into his crime."

North Korean Foreign Minister Ri Yong Ho leaves Ca North Korean Foreign Minister Ri Yong Ho leaves Capital International Airport in Beijing, China, after returning from talks in Sweden, March 19, 2018. North Korean Foreign Minister Ri Yong Ho leaves Capital International Airport in Beijing, China, after returning from talks in Sweden, March 19, 2018.

Last week's surprise Stockholm meeting between North Korean Foreign Minister Ri Yong Ho and his Swedish counterpart, Foreign Minister Margot Wallstrom, set off speculation that Sweden would be a possible location for the summit between Trump and Kim Jong Un.

The meeting also brought the issue of Tony Kim and two other Americans to the fore as Sweden is thought to be negotiating with North Korea for release of the U.S. detainees. Sweden has maintained relations with North Korea since 1973 and is one of the few Western countries with an embassy in Pyongyang. It provides consular services for the U.S. in North Korea.

However, State Department spokesperson Heather Nauert said on Wednesday "there's nothing under way" although seeking the detainees release is "a high priority for this administration."

Sol Kim said he has not heard anything from the State Department about his father's possible return.

Sol Kim and his family members and friends have been sending letters to Tony Kim via the State Department, hoping that, somehow, the letters would wind up with the Swedish Embassy in Pyongyang for delivery.

"But I think the letters have not gone [to him] … We just don't know," Sol Kim said.

FILE - a building at the Pyongyang University of S FILE - a building at the Pyongyang University of Science and Technology. FILE - a building at the Pyongyang University of Science and Technology.

Accountant turned professor

Tony Kim, a former accountant turned professor, had been in North Korea teaching international finance and management to students at Pyongyang University of Science and Technology (PUST), the only private university in the country.

He also taught at PUST's affiliate institution in China, the Yanbian University of Science and Technology (YUST) in Yanji, for more than 15 years. While at the Yanbian University, the 59-year-old professor made numerous trips to North Korea to teach at PUST after it opened in 2010.

PUST was founded by an evangelical Christian and funded from outside North Korea after the regime authorized the Northeast Asia Foundation for Education and Culture to establish PUST. The school has more than sixty foreign faculty members from China, the U.S., Canada, the U.K., and other European countries, according to its website.

Sol Kim, a 27-year-old graduate student in Southern California, visited North Korea as his father"s teaching assistant.

"I got to see students study. … I got to spend time playing sports after class time. We'd eat and share meals together," Sol Kim said. "They were very curious. They worked hard. It was a positive experience."

Athletes from North Korea and South Korea celebrat Athletes from North Korea and South Korea celebrate during the closing ceremony at the Pyeongchang 2018 Winter Olympics. Athletes from North Korea and South Korea celebrate during the closing ceremony at the Pyeongchang 2018 Winter Olympics.

The Olympics thaw

Sol Kim began speaking out about getting his dad released as tensions began thawing on the Korean Peninsula during the Winter Olympics.

When he heard about the summit between the U.S. and North Korea, Sol Kim ramped up his efforts to get his father released. He talks to the State Department every week. He's posted on YouTube and launched #USA3.

"I think the response was good. I don't know how many people read but people would repost or retweet, sharing with their friends," said Sol Kim.

"They are encouraging for me. I'm not … doing this to get millions and millions of views," he said. "But the fact that people took the time to share and hear the messages … was encouraging."

Sol Kim has messages for his father, ones he hopes reach the elder Kim … somehow: "We miss him a lot. I love him. We want him to know that he'll be becoming a grandpa soon. I look forward to seeing him again."

The last time he had word of his father was when Joseph Yun, the U.S. special representative for North Korea policy who retired early this month, visited North Korea in June 2017 to secure the release of Otto Warmbier, an American college student who died shortly after his release in a comatose state. Warmbier's death prompted Trump to issue a ban on U.S. citizens traveling to North Korea.

Two other U.S. citizens, all ethnic Koreans Kim Hak Song and Kim Dong Chul are also detained in North Korea on charges of conducting anti-state activities to overthrow the North Korean government.

Christy Lee contributed to this report which originated on VOA's Korean Service.