“They’ve been using this method for a long time,” said Zhang Jianping, a contributor to the Web site who has known Mr. Huang since 2005. Nobody knows the grounds for his arrest, but many people have the same idea. Mr. Zhang said, “It may be because the schools collapsed, and so many children died.”

In the days after the earthquake, the authorities allowed reporters and volunteers to travel freely in the disaster zone. Some commentators even saw the dawning of a Chinese glasnost. In an interview with National Public Radio that aired in May, Mr. Huang said he believed that the human rights situation in China had greatly improved.

“He actually thought things were heading in the right direction,” said John Kamm, who is pressing for Mr. Huang’s release and is the executive director of the Dui Hua Foundation, which has helped free prominent Chinese political prisoners. “That’s one of the tragedies of his detention.”

A volunteer at the Tianwang center, Pu Fei, 27, was detained minutes after Mr. Huang. He said that the officers who interrogated him demanded that he hand over the password needed to post information on their Web site. They also wanted to know whom Mr. Huang had met and where he had gone in the disaster zone. Mr. Pu was detained in a hotel for two weeks and then released.

Image Protesting parents in Mianzhu clashed with the police on May 25. There is no official death toll for students. Credit... Shiho Fukada for The New York Times

Mr. Pu and other volunteers said the authorities might have singled out Mr. Huang because he disseminated information about parents whose children had died in collapsed schools  a group whose protests began to snowball into something like a movement in early June.