Four people were killed and another four were badly injured when a high-speed railway tunnel being constructed in Guizhou province collapsed last Tuesday, work safety officials said on Thursday morning.

The Dadu Mountain tunnel cave-in occurred at 11 am on Tuesday, trapping workers who were tasked to finish the high-speed railway tunnel on time, authorities said.

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According go the Guizhou Provincial Work Safety Administration, rescue efforts were called on Wednesday after rescuers pulled out the bodies of four workers and brought four injured persons to the local hospital.

As of Thursday morning, the four injured workers were declared out of danger by attending physicians. The case of the tunnel collapse continues to be investigated, authorities said.

The tunnel is part of the high-speed railway line that connects China’s Shanghai and Yunnan Province’s capital Kunming, a major component of the Chinese government’s efforts to construct more high-speed railway lines to speed up the flow of goods within the country.

Last March, the government has unveiled the longest railway tunnel in northwest China, the 22.449-kilometer left part of the Zhongtianshan Tunnel. The tunnel is the longest of its kind in Xinjiang Uygur Autonomous Region.

The Urumqi Railway Bureau (URB) said the tunnel, constructed by the URB and the 18 Bureau Group Co, is also the third-longest in the country. It took about seven years to complete the entire tunnel, the URB said.

“The right section of the tunnel, which is about 22.47-kilometer long, was completed ahead in September 2013,” said the URB in a statement posted on its website.

The Zhongtianshan Tunnel is a major component of the Nanjiang Railway that connects Korla in southern Xinjiang and Tupran. The entire 333.87-kilometer railway line is set to start operation before the end of 2014.

The URB said the Nanjiang Railway will slash travel distance by about 122 kilometers.