The explosion occurred July 20 inside the project's headrace tunnel. Project documents issued 10 days before the explosion identified methane gas, water ingress, cavity formation and unstable ground as active construction hazards in the same section where the accident occurred. There is no indication whether those measures were in place when the explosion occurred. The precise sequence—including whether the gas explosion caused the tunnel to collapse or was compounded by one—has not yet been publicly established. According to the news service, 21 laborers were working underground when the explosion occurred, while six project officials entered during the initial response.