Two construction workers died and another was hospitalized after being overcome by gas while working inside a septic tank at a residential property in Vietnam’s capital Hanoi on Thursday, local media Thanh Nien reported.

The accident occurred in Da Phuc commune on Thursday morning when three workers entered the underground pit to remove formwork and were overcome by gas, according to the report.

Nearby residents rushed to pull the victims from the confined space, but two of them had died, while the third was taken to hospital and was in stable condition.