A review of Ms. Ma’s case — involving interviews with officials, diplomats and others, as well as searches in corporate registries — underscored China’s deep ambivalence as it has come under increasing pressure to enforce sanctions against North Korea. While China is on the record opposing the North’s pursuit of nuclear weapons, it is wary of being seen imposing punishments at the bidding of the United States, especially against its own citizens.

Image Ms. Ma in a company photo. Since China announced an investigation in 2016, her whereabouts has been shrouded in mystery.

North Korea’s agreement on Tuesday to send athletes to the Winter Olympics in South Korea next month, and to hold talks and other exchanges with the South, may have been symbolic and perhaps a cynical effort to bide time. Yet it suggested that the rising diplomatic and economic pressure, meant to deny it the financial and material resources needed to develop nuclear weapons and ballistic missiles, may have had some effect on the North’s leader Kim Jong-un.

Mr. Kim has given no signal that he would give up his nuclear ambitions, but after the North’s initial overtures, Mr. Trump wrote on Twitter that the talks were evidence that “sanctions and ‘other’ pressures are beginning to have a big impact on North Korea.”

China has shown a willingness to support tougher sanctions at the United Nations Security Council over the last year, but it has done so grudgingly. The reasons for that are historical and strategic. North Korea has long counted on China as its only real ally, for example, but some analysts argue that economic factors also play a part.