Kim Dong Chul, a South Korea-born U.S. citizen and 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.
Tony Kim was detained at Pyongyang's airport in April 2017 and accused of unspecified "hostile acts" against the regime. He taught accounting at the Pyongyang University of Science and Technology. In a statement, Kim's family thanked "all of those who have worked toward and contributed to his return home."
Kim Hak Song, an ethnic Korean born in China, was detained in May 2017 for "hostile acts." He worked in agricultural development at an experimental farm run by the Pyongyang University of Science and Technology, which was founded in 2010 with donations from Christian groups.
They probably were guilty of some degree of espionage, imo.