A Chinese filmmaker detained while making a documentary about underground Christians was released Tuesday after more than five months in police custody, his sister said in an Internet posting.

Wu Hao was taken away by authorities Feb. 22, and police repeatedly refused to give details about charges or let his family see him, saying his case had to be kept secret.

Before his release, police had informed the filmmaker he was under "supervised residence," according to a Web diary kept by his sister, Wu Na.

In a short notice on Wu's blog Tuesday, she wrote that she had received a call informing her that her brother was free. Reached by phone, Wu said it wasn't convenient for her to talk to the press at this time.

"Just got a call at home and was informed that Wu Hao is out," Wu wrote. "Thank you everyone for your concern, but he needs some silence for now. If there is any new information it will be posted on this blog."

Chinese authorities often use "supervised residence" to hold people at small hotels in isolated locations under constant surveillance.

The Paris-based press freedom group Reporters Without Borders issued a statement saying it was "immensely relieved" to learn of Wu Hao's release. The group had been lobbying China's central government to intervene in the case.

Wu Hao lived in New York, Massachusetts and California for 12 years before returning to China in 2004 to make documentaries. At the time of his detention, he had been working on a film about Christian congregations in China that worship outside the state-controlled church.

Wu was also linked to Gao Zhisheng, a Beijing lawyer who has run afoul of authorities for mounting spirited defenses in politically sensitive cases.