There were unconfirmed reports on Thursday that North Korea has moved U.S. nationals from prisons in the isolated communist country to a hotel in or near the capital ahead of an expected meeting between President Donald Trump and North Korean leader Kim Jong Un. U.S. officials have not confirmed that the three men are out of North Korea's notorious prison camps, but Rudy Giuliani, who recently joined the White House legal team, told Fox News on Thursday that the North was "releasing three prisoners today."

It was not immediately clear whether Giuliani had confirmation that U.S. citizens' release was imminent, and White House Press Secretary Sarah Huckabee Sanders said Thursday that, while "we think that that 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," she could not make any "announcement" about the U.S. detainees.

As everybody is aware, the past Administration has long been asking for three hostages to be released from a North Korean Labor camp, but to no avail. Stay tuned! — Donald J. Trump (@realDonaldTrump) May 3, 2018

Mr. Trump has said his administration is "fighting very diligently" to see the three men released, and he hinted on Twitter that more information might be coming.

"As everybody is aware, the past Administration has long been asking for three hostages to be released from a North Korean Labor camp, but to no avail. Stay Tuned!" he said.

Below is a brief explanation of who the three American detainees are, and the circumstances under which they ended up imprisoned in North Korea -- to the extent they are known.

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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.

Undated photo from North Korean state TV showing Kim Dong Chul, one of three Americans held in North Korea. KCNA via Reuters

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 said in May 2017 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, who also uses the name Kim Sang-duk, was detained a year ago at the Pyongyang airport. He taught at the Pyongyang University of Science and Technology, just like Kim Hak Song.

Family photo of Tony Kim, one of three Americans being held in North Korea. Family of Tony Kim via Reuters

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

In April 2017, the chancellor of the university said Kim, who is in his 50s, taught accounting at the university for about a month and had taught at a university in China before coming to Pyongyang. 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 confirmed the detention of Kim in May 2017.