right now a u.s. citizen is being detained by north korea. he is the third american held by the regime. it is raising tensions even more as an american carrier strike group approaches the region. cnn's ivan watson is live in seoul, south korea, with more. ivan, what do we know? >> reporter: okay, this is a professor by the name of tony kim, chris. he had been teaching for several weeks at the pyongyang university of science and technology. he was on the way out of the country leaving saturday morning from pyongyang international airport and that is when the north koreans grabbed him. we don't know why and we don't know the charges they may press against him. once the north koreans do this, judging by past behavior and trends, it is very, very serious. the other two americans that are