HONG KONG — Before she went on holiday with her boyfriend, the young woman from Hong Kong told friends about his pledge to her. “He said I am his first and last girlfriend,” the woman, Poon Hiu-wing, 20, wrote on Facebook. It would be her last Facebook post.

After their trip to Taiwan in February 2018, the boyfriend, Chan Tong-kai, returned alone. Her family grew worried and called the police. One week later, investigators found her body in the bushes near a light rail station in northern Taiwan. An autopsy determined that she had been four to five months pregnant.

The police in Hong Kong arrested Mr. Chan, and last year he was charged over possession of his girlfriend’s cellphone, digital camera and $3,000 in cash.

But he was not charged with killing Ms. Poon.

Under Hong Kong law, Mr. Chan must face charges related to her death where it happened, in Taiwan. But Hong Kong does not have an extradition agreement with Taiwan, so Mr. Chan has remained in Hong Kong.