WASHINGTON -- Secretary of State Rex Tillerson said Tuesday that North Korea had released Otto Warmbier, an American serving a 15-year prison term with hard labor for alleged anti-state acts.

Tillerson said in a statement that Warmbier is on a Medivac flight on his way back to the U.S. to be re-united with his family. He said the State Department secured Warmbier's release at the direction of President Trump. Tillerson said the State Department was still discussing two other detained Americans with North Korea.

Fred and Cindy Warmbier told The Associated Press that they have been told their son has been in a coma since March 2016, and they had learned of this only one week ago.

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"We want the world to know how we and our son have been brutalized and terrorized by the pariah regime," they said.

Otto Frederick Warmbier (C), a University of Virginia student who was detained in North Korea since early January, is taken to North Korea's top court in Pyongyang, North Korea, in this photo released by Kyodo on March 16, 2016. Reuters

The couple also said they are grateful he "will finally be with people who love him."

The State Department announced Warmbier's release earlier Tuesday but gave no details on his condition.

Tillerson's announcement came as former NBA player Dennis Rodman paid a return visit to North Korea. Rodman said ahead of his trip that the detained Americans were not on his agenda, his fifth to the country but his first with President Trump in office.

Diplomats at the U.N. on Tuesday expressed dismay that Rodman arrived in North Korea at a delicate and dangerous time for the countries threatened by the government in Pyongyang, since his past trips have produced nothing more than positive publicity for the repressive government of Kim Jong Un, CBS News' Pamela Falk reports.

Warmbier is a University of Virginia student from suburban Cincinnati. He was sentenced in March 2016 after a televised tearful public confession to trying to steal a propaganda banner.

CBS News correspondent Seth Doane reported last year that Warmbier was in North Korea for a five-day visit and was detained at Pyongyang's airport ahead of a flight back to China.

Kim Dong Chul, who was born in South Korea but is also believed to have U.S. citizenship, is serving a sentence of 10 years for espionage in the North, and in April this year, Tony Kim, who also goes by his Korean name Kim Sang-duk, was detained according to the Pyongyang university at which he was working.

In previous cases, people who have been detained in North Korea and given a public confession often recant those admissions after their release.

Warmbier was arrested while visiting the country with Young Pioneer Tours, an agency specializing in travel to the North, which is strongly discouraged by the U.S. State Department. He had been staying at the Yanggakdo International Hotel, which is located on an island in a river that runs through Pyongyang.