BEIJING (AP) — The bishop of an unregistered Roman Catholic church in northern China has returned home, after being taken away by authorities on the last day of the Beijing Olympics, a U.S.-based monitoring group said Friday.

However, Bishop Jia Zhiguo remained under 24-hour police surveillance at his home at the Christ the King Cathedral in Wuqiu village of Hebei province, the Cardinal Kung Foundation said in a statement.

Jia, who is in his 70s, was barred from receiving visitors after police escorted him home Thursday and it was not known how he was treated during his detention, said the foundation, which has close contacts with China's unregistered church members.

Jia has been repeatedly detained by security forces in China, which broke ties with the Vatican in 1951 and demands that Catholics worship only in government-controlled churches. Such churches recognize the pope as a spiritual leader but appoint their own priests and bishops.

Millions remain loyal to the pope and worship in unregistered churches, but priests and members of their congregations are frequently detained and harassed.

It was not known why Jia was detained this time, though Chinese authorities took away numerous dissidents and activists during the Olympics and Paralympics for disabled athletes. The Paralympics ended Wednesday, one day before Jia was released.

The Chinese government wanted to use the games to showcase China's emergence on the world stage, and was on high alert against public disturbances during the events.

A woman in the news office of the public security bureau in Jinzhou city, which oversees Wuqiu village, said she had never heard of Jia's case but would look into it.

Jia's Zhengding diocese, 150 miles southwest of Beijing, is a traditional stronghold of Catholic sentiment in northern China.