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.”

The explosion on Tuesday caused part of the 10-story underground structure to collapse, flooding several levels, which restricted rescue efforts. The blast occurred eight floors below ground, and smoke and high temperatures from a subsequent fire were initially also a challenge for emergency workers, said Mr. Cari, the firefighters’ spokesman.

The plant was undergoing what the company described in a news release as “efficiency works” to update it. The work began in September 2022, and Enel Green Power said it used contractors known to be experts in the field. The workers killed and injured by the explosion were carrying out final tests on the plant’s two power-generating modules. One module was successfully tested in recent weeks. The explosion occurred when testing of the second module “was underway,” the company said.

“The work being done here can only be done by specialists,” Salvatore Bernabei, chief executive of Enel Green Power, told reporters in a video interview broadcast by Sky News Italia. “We chose the most prestigious companies that carry out this kind of work,” he added.

Michele Bulgarelli, the president of the Bologna branch of the trade union CGIL, described the incident as the “worst massacre of workers” in the area in recent memory. He criticized Enel Green Power for its “lack of transparency,” saying the company had not released to the unions the names and roles of the people involved in the accident.

“Yesterday we were dismayed; today we are angry,” he added.

The hydroelectric power plant is the most powerful in Emilia Romagna, one of Italy’s most industrious regions. Built in 1975, it is run by Enel Green Power, the Enel Group’s renewable energy branch. The company said that the dam basin on Lake Suviana, an artificial lake formed by the construction of the dam in 1928-32, had not been damaged and was considered safe. There was “no impact on the supply of electricity locally and nationally,” the company said.

PierPaolo Bombardieri, national secretary of the Uil trade union, said in a statement on Wednesday that in 2022, local representatives of his union had “raised some questions of safety issues for that facility.” He added that the union was available to give investigating prosecutors “information and documentation on the case.”

Trade unions in Emilia Romagna announced that in that region, a four-hour nationwide strike that was already scheduled for Thursday, would be extended to eight hours and would involve both the private and public sectors, to protest work-related deaths.

According to Inail, the National Institute for Insurance Against Accidents at Work, which monitors work-related accidents and deaths, there were more than 1,000 work-related deaths in Italy in 2023.

Matteo Lepore, mayor of Bologna, the capital of the Emilia Romagna region, called on residents to participate in the march organized by the trade unions that would snake through the city center Thursday morning. “We need a big demonstration to say enough to work-related deaths and be close to the colleagues and families” of the workers involved in the blast, he said, adding, “We must be there tomorrow.”

The accident also drew lawmakers — both local and national — to the scene.

“Another massacre of workers, we can no longer accept that this occurs, we need to make safety at work become the priority in this country,” said Elly Schlein, the leader of the opposition Democratic Party, who spoke to reporters at the site of the accident on Wednesday. She described it as “an immense tragedy.”

On this occasion, she saw eye to eye with Italy’s labor minister, Marina Elvira Calderone, who also traveled to the plant.

“We have to invest in a culture of safety,” the minister said, adding that laws existed, and others were in the works.

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