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.