According to Taoyuan City police, the victims - two men and two women, aged between 20 and 27 - were discovered on May 3 in an apartment located in Yangmei District, Taiwan News reported.
The bodies were found lying inside the room with no signs of struggle or external injuries. Authorities noted the apartment's doors and windows were shut, and hot water was running in the bathroom.
Police identified two of the deceased as undocumented workers, one a student, and the other a legally employed worker, according to the Taipei Times.
Preliminary investigations suggest that carbon monoxide poisoning may have caused their deaths. Autopsies have been ordered to confirm the exact cause, while Vietnam's representative office in Taiwan has been notified about the case.