"He has said the case against him was retaliation for his political activism, including his memorializing the thousands of children who died in schools that collapsed during a 2008 earthquake in Sichuan Province.

"He said on Wednesday that the authorities had given him no indication of why he had received his passport now. 'I only can say why not? They have promised for the past four years to give it back. Now finally they gave it to me,' he said in a telephone interview. 'They always say it's in the process but I just need to be patient.'

"The confiscation of his passport meant that Mr. Ai was forced to organize his overseas exhibitions remotely, including shows at the Hirshhorn Museum and Sculpture Garden, part of the Smithsonian Institution in Washington, and at Alcatraz, the former prison in San Francisco Bay."