The Garratts have lived in China on and off since 1984 and moved to Dandong about six years ago, said their son Simeon Garratt, 27, who returned to Canada in 2009. They run Peter’s Coffee House, named for their younger son; the restaurant is known for its Western fare and for a discussion group for people studying English. “It’s a great opportunity to learn,” Liu Ziyu, who frequently attended the discussion group, said by phone.

The coffee shop became a hub for the city’s expatriate English teachers and other foreigners, some of whom had come with religious ambitions.

The Garratts made no secret of their Christian faith, which drew them to Dandong. In a sermon posted on the website of the Terra Nova Church in Surrey, British Columbia, Mr. Garratt, 54, framed the couple’s life in China in biblical terms: “God said, in a prayer meeting, go to Dandong and I’ll meet you there, and he said start a coffee house.” He added, “We’re trying to reach North Korea with God, with Jesus, and with practical assistance.”

In Dandong, the Garratts worked with North Star Aid, a humanitarian group focused on delivering food to North Koreans. According to the sermon, dated last November, they were cooperating with house churches in China to send clothes and other aid to North Korea and operated a special “training house” outside Dandong for North Koreans who crossed the border.

China looks askance at those seeking to assist North Koreans in the highly sensitive region. “If this couple was deeply involved in the North Korean refugee issue and then sending people back in to do missionary work, that is not something the Chinese want foreigners actively engaged in on that border,” said John Delury, an assistant professor of Chinese and East Asian studies at Yonsei University in Seoul, South Korea.

The couple’s younger son, Peter, 21, said they had been careful not to violate government restrictions on proselytizing. “They don’t try to hide it, but they’re not doing anything against the law,” he said by phone from Dandong.

He said he had last spoken to his parents on Monday night, while they were out for dinner.

On Tuesday, Peter was summoned for questioning by the state security bureau and told to pick up some clothes and toiletries for his parents, he said.

Reached by phone in Vancouver, Simeon, one of three Garratt siblings back in Canada, said: “My parents have loved China ever since they visited on their honeymoon. If you knew them, it would be the most absurd thing you could ever imagine.”