North Korea says it has detained a US citizen on suspicion of "hostile acts" against the state.
Kim Hak-song worked at the Pyongyang University of Science and Technology (PUST) and was held on 6 May, state-run news agency KCNA said.
Three other US citizens are currently held in North Korea, including Kim Sang-duck, who had taught at PUST.
The US has in the past accused North Korea of detaining its citizens to use them as pawns.
KCNA said that "a relevant institution" was "conducting [a] detailed investigation" into Kim Hak-song's alleged crimes.
It gave no further details.
A State Department official said Washington was "aware of reports that a US citizen was detained in North Korea", adding it would liaise with the Swedish Embassy in Pyongyang, which helps look after US interests in North Korea.
Kim Hak-song had previously described himself as a Christian missionary who intended to start an experimental farm at PUST, Reuters news agency reported, citing an online post by Mr Kim.