Friends say Chinese police are expanding their investigation into missing contemporary artist Ai Weiwei.
American filmmaker Alison Klayman said that police are questioning more people who know Ai, who was last seen on Sunday in police custody at a Beijing airport.
Police have also searched his home and studio.
A police spokesman said he had no information on Ai's case.
Ai's disappearance comes as China carries out a massive crackdown on lawyers, writers and activists, following online calls for protests similar to those in the Middle East and North Africa. No public protests have emerged.
Ai is a government critic who has been keeping an informal tally of recent activist detentions.
Klayman has been working on a documentary about Ai.