GUWAHATI, India (AP) — A suspected gas explosion collapsed part of a tunnel under construction at a hydropower project in India’s northeastern Himalayan state of Sikkim, leaving at least 12 people dead and 13 missing, officials said Tuesday.

Rescue teams with oxygen gear raced to find survivors Tuesday as toxic levels of gas were detected at the disaster site in Samardung village near the Chinese border.

The blast had ripped through the tunnel Monday while 21 laborers were inside working on the 500-megawatt Teesta hydroelectric project, officials said. Two of them managed to escape. Six power utility officials later entered the tunnel to help in the rescue, but became trapped themselves, officials said.

The explosion was believed triggered by a buildup of methane, said State Chief Minister Prem Singh Tamang, who visited the site Tuesday. Survivors told authorities they heard a loud explosion before debris came crashing through the tunnel, civil official Anupama Tamling said.

Officials had recovered 12 bodies on Tuesday while rescuers from the National Disaster Response Force, or NDRF, raced to reach the 13 people believed to be still trapped.

“We found toxic gases such as methane, carbon monoxide and hydrogen sulfide inside the tunnel,” NDRF official Nitin Kumar said at the scene.

The conditions late Tuesday were so dangerous that, although equipped with oxygen gear, the rescuers could enter the disaster site only for shifts of about 15 minutes at a time, officials said.

Construction accidents are not uncommon in India, where rapid infrastructure growth often collides with fragile geology, construction lapses, extreme weather and safety standards.

In the Himalayan region, fragile mountains, seismic activity and unpredictable underground conditions make tunnel projects especially risky. An explosion inside a coal mine in neighboring Meghalaya state in February killed at least 18 workers. In 2023, a tunnel section in northern Uttarakhand state collapsed, trapping 41 workers for 17 days before they were rescued.

Experts say the Teesta River basin, where the hydropower project is being built, lies in a seismically active region marked by young, fragile rock formations riddled with fractures and underground pockets that can trap ancient gases.

Dr. Devesh Walia, a geology professor at North-Eastern Hill University in Meghalaya, said the area contains relatively young rock formations, some associated with coal-bearing strata.

“It is quite possible that methane along with other gases accumulated underground and may have triggered an explosion,” he said.