BEIJING—Chinese authorities are investigating Ai Weiwei for unspecified "suspected economic crimes," state media reported, in the first official acknowledgment of the famous artist-activist's status since he disappeared into government custody.

The one-sentence dispatch from the official Xinhua news agency early Thursday in Beijing, which cited "police authorities," gave no details about the investigation or the whereabouts of Mr. Ai, a harsh critic of the government who has been out of contact since being stopped and led...