Complex sentence: At that point in time, state mine inspectors were in the Centralia-area mines almost daily to check for lethal levels of carbon monoxide, which was found on August 9, signaling the closing of all Centralia-area mines the next day.
Split into simpler sentences: At that point in time, state mine inspectors were in the Centralia-area mines almost daily to check for lethal levels of carbon monoxide. Lethal levels were found on August 9, and all Centralia-area mines were closed the next day.