As recently as Thursday, the men—two from China and one from Myanmar—were still believed to be alive.
On Saturday, a section of the tunnel they were constructing collapsed in Pak Chong district, some 200 km (124 km) northeast of Bangkok, trapping them.
Authorities declared all three dead on Friday, and preliminary findings point to a lack of oxygen as the cause.
A landslide caused the tunnel, which was under construction and was a component of the high-speed railway project between Thailand and China, to collapse last Saturday at approximately 23:40 local time (16:40 GMT).
In an attempt to save the victims, rescuers—who included members of the Chinese disaster response team and representatives of the State Railway of Thailand (SRT)—had been working nonstop for the past week.
It’s not known if the tubes they used even made it to the workers, but they had attempted to pump oxygen into the tunnel to keep them alive.
Around 6:00 local time (00:00 GMT) on Friday, the bodies of the two Chinese workers—a supervisor and an excavator operator—were discovered, according to the SRT.
