Specialized search and rescue teams, underwater divers, cave experts and topographers were searching on Wednesday for four workers who were missing a day after an explosion at a hydroelectric plant near the northern Italian city of Bologna killed at least three people and injured five others.

“It’s a complicated situation,” Luca Cari, a spokesman for Italy’s firefighters, said. The explosion took place at a level of the plant that was underwater, and Mr. Cari said divers on Wednesday worked in conditions of “zero visibility,” as they searched among the rubble and debris of the explosion, moving sheet metal by hand with difficulty.

The explosion at the Enel Green Power plant happened as the company was testing efficiency improvements that had been made to the facility, which generates power from the water of a nearby dam basin.

The episode and the resulting deaths and injuries roused unions to call for a general strike protesting unsafe working conditions and work-related deaths. Political leaders, both in the center-right government and in the opposition, rushed to the site of the plant, about 43 miles south of Bologna.

Prosecutors in Bologna said Wednesday that they would open an investigation into the possible causes of the explosion. Enel Green Power said in a statement that it “would continue to fully collaborate with the relevant authorities to ascertain the facts.”