A North Korean university has named the US citizen detained on Saturday as Kim Sang-duk, also known as Tony Kim.
The Korean-American lecturer taught at Pyongyang University of Science and Technology (PUST) for several weeks prior to his arrest.
The investigation into Mr Kim was for matters “not connected in any way” to the university, PUST said in a statement to the BBC.
Mr Kim was arrested just as he was about to leave Pyongyang.
North Korean authorities have not yet disclosed the reason for the arrest.
According to South Korean news agency Yonhap, Mr Kim, who is in his late 50s, was involved in aid programmes and had been in North Korea to discuss relief activities.
He reportedly taught at Yanbian University of Science and Technology in China, which is affiliated to PUST. The BBC’s calls to Yanbian were not answered.
PUST’s chancellor, Park Chan-mo, was quoted by Reuters news agency as saying that Mr Kim “had been involved with some other activities outside PUST such as helping an orphanage”.
The US State Department said it was aware of reports of the detention, but would not commented further because of “privacy considerations”, US media reported.