President Trump announced early Wednesday morning on Twitter that Secretary of State Mike Pompeo was on his way back to the U.S. from North Korea with three Americans who had been released from prison by the isolated totalitarian state. Mr. Trump had hinted over the past week that the three men would soon be released, but only Tuesday did he announce Pompeo's trip to North Korea -- ostensibly to work out the details of his upcoming summit with Kim Jong Un.

"They seem to be in good health. Also, good meeting with Kim Jong Un. Date & Place set," Mr. Trump added in his tweet.

I am pleased to inform you that Secretary of State Mike Pompeo is in the air and on his way back from North Korea with the 3 wonderful gentlemen that everyone is looking so forward to meeting. They seem to be in good health. Also, good meeting with Kim Jong Un. Date & Place set. — Donald J. Trump (@realDonaldTrump) May 9, 2018

Mr. Trump said Pompeo and his "guests" would land at Joint Base Andrews, outside Washington D.C., at 2 a.m. Thursday, and that he would "be there to greet them. Very exciting!"

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In a written statement, the White House said "President Trump appreciates leader Kim Jong Un's action to release these American citizens, and views this as a positive gesture of goodwill. The three Americans appear to be in good condition and were all able to walk on the plane without assistance."

Vice President Mike Pence issued a statement on Wednesday crediting "President Trump's tough minded diplomacy" with winning the release of the three men, saying the administration was "encouraged" by it, but warning, "we will not let off the pressure until we achieve full denuclearization" of North Korea.

Secretary Pompeo and his “guests” will be landing at Andrews Air Force Base at 2:00 A.M. in the morning. I will be there to greet them. Very exciting! — Donald J. Trump (@realDonaldTrump) May 9, 2018

The longest-imprisoned of the three U.S. nationals, Kim Dong Chul, was sentenced in April 2016 to 10 years hard labor for espionage. The two others -- Kim Hak Song and Tony Kim -- were working at the Pyongyang University of Science and Technology when they were detained about a year ago.

The university said Wednesday that it was, "very grateful to learn of the release of the three US-Korean citizens," adding that "all three men have been daily in our thoughts; and our hopes and prayers have been fulfilled by their release."

Tony Kim's family thanked Mr. Trump in a statement released Wednesday morning, saying: "We are very grateful for the release of our husband and father, Tony Kim, and the other two American detainees. We want to thank all of those who have worked toward and contributed to his return home. We also want to thank the President for engaging directly with North Korea. Mostly, we thank God for Tony's safe return."

Three American detainees being released from North Korea

Their release comes as Mr. Trump and North Korean leader Kim Jong Un prepare to hold a summit -- possibly within weeks. Kim and South Korean President Moon Jae-in held a historic meeting in the Demilitarized Zone on April 27, agreeing to work toward denuclearizing the Korean Peninsula and discussing plans to declare a formal end to the Korean War.

Pompeo confirmed in a tweet of his own later Wednesday morning that he had met with Kim while in Pyongyang. He said he had "productive meetings" with the North Korean leader, but did not confirm Mr. Trump's earlier tweet saying the time and place for the summit had been confirmed.

I had productive meetings in Pyongyang with Chairman Kim Jong-un and made progress. I'm delighted to bring home three Americans. — Secretary Pompeo (@SecPompeo) May 9, 2018

Very little is known about why the three men were actually detained by North Korea, or what conditions they faced in custody. Below is a brief summary of what information was made available on the three men.

Kim Dong Chul

Kim, a South Korean-born U.S. citizen, has been held the longest. The former Virginia resident was sentenced in April 2016 to 10 years in prison with hard labor after being convicted of espionage.

He reportedly ran a trade and hotel service company in Rason, a special economic zone on North Korea's border with Russia.

Kim Hak Song

Kim worked in agricultural development at an experimental farm run by the Pyongyang University of Science and Technology. The university is the only privately funded college in North Korea and was founded in 2010 with donations from Christian groups. He was detained last May for alleged anti-state activities.

The North's official Korean Central News Agency announced that Kim was detained and that "a relevant institution is now conducting detailed investigation into his crimes."

He worked at Pyongyang University of Science and Technology.

Tony Kim

Tony Kim, 59, who also uses the name Kim Sang-duk, was detained a year ago at the Pyongyang airport. He had been teaching at the Pyongyang University of Science and Technology, as had Kim Hak Song.

He was accused of committing unspecified criminal acts intended to overthrow the government. The North has not said whether the two cases of staff at the university are connected.

In April 2017, the chancellor of the university said Kim taught accounting at the university for about a month and previously taught at a university in China. He said he was informed that the detention had "nothing to do" with Kim's work at the university, but he did not know anything further.

North Korea only officially confirmed the detention of Kim in May 2017.

Kim's son, Sol Kim, told CBS News he was stunned by his father's detention. "I asked myself why, what happened, what is the reason, lots of different thoughts went through my head. Will I see him again?" he said. "Part of me really thought he did not do anything wrong, he won't be held very long."

Lead up to release

There were unconfirmed reports that the detainees were moved last week from prisons to a hotel in or near the capital, Pyongyang. But asked on May 3 about their possible release, White House Press Secretary Sarah Huckabee Sanders hedged, saying it "would be an incredible sign of goodwill, and certainly a great statement for the North Koreans to make ahead of the summit and the discussions," but that she had no announcement at the time.

Speaking to reporters on Tuesday, Pompeo said he'd be meeting with senior North Korean leaders but didn't know if he'd meet with Kim. "We're prepared to meet with anyone who is speaking on behalf of the North Korean government and can give us solid answers so we're prepared," he said.

"We have been asking for the release of these detainees for this administration for 17 months," he told reporters on Tuesday. "We'll talk about it again today. I think it'd be a great gesture if they would choose to do so."

Pompeo secretly met with Kim in April while he was still director of the CIA. He was confirmed and sworn in as secretary of state on April 26 and quickly departed on his first foreign trip to Europe and the Middle East.

The release of the three Americans comes 11 months after another American detainee, 22-year-old college student Otto Warmbier, was sent home from North Korea in a coma. Warmbier never recovered consciousness and died a week later.

Warmbier was detained in January 2016 while visiting North Korea at the end of a five-day tour. He was later sentenced to 15 years hard labor for allegedly stealing a propaganda banner at his hotel.