Three people have died and six are missing following an explosion at a hydroelectric power station near Bologna in central Italy on Tuesday, a local official told AFP.

Three people were also injured in the incident at the Bargi power plant, which is run by Enel Green Power located on Lake Suviana in Italy’s Apennine Mountains, according to the official from the Bologna prefecture. Firefighters rescued the three injured, and officials said they would continue to search for missing people.

Italian firefighters shared an aerial video on social media Tuesday that showed smoke rising from underground at the power plant. In a translated post shared with the video, firefighters said the three injured people were workers and confirmed that search operations were ongoing to find anyone else potentially unaccounted for.

The mayor of the nearby town of Camugnano, Marco Masinara, said the explosion happened nine levels below ground — a depth of about 100 feet — during work on turbines, according to the Italian news outlet il Resto di Carlino. Masinara indicated that there were challenges in reaching the victims.

The partially submerged hydroelectric plant is on an artificial lake, formed by the construction of a dam between 1928 and 1932 in a regional park. The dam was not damaged, Italian wire services reported, but flooding into the affected part of the plant meant rescue missions were difficult and dangerous.

Authorities said it was impossible to know what exactly caused the explosion. A team, believed to be doing maintenance on the turbines at the center of the plant, was working on the ninth level below ground when the blast happened.

The Associated Press contributed to this report.