By Joshua Berlinger, CNN

(CNN) -- Three American citizens held for months in North Korea have been released, US President Donald Trump announced Wednesday.

Kim Dong Chul, Kim Hak-song and Kim Sang Duk were freed during US Secretary of State Mike Pompeo's second visit to the North Korean capital of Pyongyang in two months.

The detainees are returning to the United States as Trump prepares to make history by becoming the first sitting US President to meet face-to-face with a North Korean leader.

Trump's administration had previously said that if the North Koreans freed the three Americans, it would be viewed as a goodwill gesture ahead of the planned summit between Trump and North Korean leader Kim Jong Un.

Some experts worried about the optics of a meeting being held with a leader whose country had imprisoned American citizens on what many believe were bogus charges.

Here is what we know about the three. Though they're all surnamed Kim, they are not related.

Kim Sang Duk, who also goes by the name Tony Kim, was detained last April during a brief teaching stint at the Pyongyang University of Science and Technology (PUST), which bills itself as the only privately run university in the North Korean capital.

Kim was boarding a flight at the airport in Pyongyang on April 22, 2017 when North Korean authorities detained him, according to the University. He was later accused of attempting to overthrow the North Korean government.

Tony Kim had been teaching accounting and business at PUST, his son Sol Kim said. Sol, 27, told CNN last month his father would play sports with students and had a good relationship with them, something he observed firsthand during a previous trip he made to Pyongyang as his father's teaching assistant.

Sol has been trying to raise awareness of the three by speaking to media and launching an awareness campaign titled "Free USA 3."

Speaking again to CNN this week, Sol said his brother's wife is expected to give birth at the end of July. Tony Kim doesn't know he will soon become a grandfather.

Sol said he's thankful that President Trump's administration is publicly bringing up his father's case, but as time goes by the stress of the situation is wearing him down.

"Maybe now that we're kind of close to the end ... the last few steps for him to be released seems to be going a lot slower than the first year, the whole year," said Sol Kim.

Kim Hak-song, another US citizen who was working at PUST, was detained on May 7, 2017.

Kim is an agricultural expert who'd been teaching rice-growing at the university, his wife told CNN shortly after he was detained. She said Kim was innocent and merely trying to help North Koreans feed themselves.

The country has for years struggled with famine and food insecurity.

"I believe 100% that you served our people with love," she said in an interview last May. "I hope you can stay strong there and hope you can return to our family very soon."

Kim, a naturalized US citizen, was born in Jilin, China, one of the provinces that borders North Korea, and is ethnically Korean. He was educated at a university in California. Two people who said they studied with Kim in the US described him as being committed to improving North Korea's agriculture and economy.

"Professor Kim was a man who would call North Korea as his own country. He went to Pyongyang to devote himself to the development of North Korea's agricultural technology so that the North can be self-sufficient with food," said David Kim, his former classmate.

Kim Hak-song was also ordained as an evangelical Christian pastor affiliated with the Oriental Mission Church in Los Angeles, which could have presented problems in the atheist country. While it's unclear if Kim's faith had anything to do with his detention, other Americans have been severely punished for acts Pyongyang views as proselytizing.

Kim Dong Chul, a naturalized US citizen who used to live in Virginia, was arrested in October 2015 on spying charges.

He was meeting a source to obtain a USB stick and camera used to gather military secrets, he told CNN in an interview in Pyongyang in January 2016, while in North Korean custody. The conversation was conducted in Korean through an official government translator and in the presence of a North Korean official.

The translation was later independently corroborated by CNN, but it's impossible to say if Kim was speaking under duress.

Kim said he moved to Yanji, a city near the Chinese-North Korean border that acts as a trade hub between the two countries, in 2001 and commuted daily to the North Korean special economic zone of Rason.

He said he started working as a spy in April 2013 on behalf of "South Korean conservative elements," bribing local residents to "gather important materials," which he smuggled into China or South Korea.

Kim was also tasked with taking photographs of military secrets and helping to spread anti-North Korean propaganda, he said in the interview.

After a one-day trial, North Korea sentenced him to 10 years' hard labor on espionage charges in April 2016.