BRUMADINHO, Brazil — Firefighters on Monday carefully moved over treacherous mud, sometimes walking, sometimes crawling, in search of survivors or bodies, four days after a dam collapse that buried mine buildings and surrounding neighborhoods with iron ore waste.

The confirmed death toll rose to 58, with up to 300 people still missing, authorities said. In an ominous sign, nobody was recovered alive Sunday, a stark difference from the first two days of the disaster, when helicopters were whisking people from the mud.

The slow speed of search efforts was due to the treacherous sea of reddish-brown mud that surged out when the mine dam breached Friday afternoon. It is up 24 feet (8 meters) deep in some places, and to avoid the danger of sinking and drowning searchers had to carefully walk around the edges or slowly crawl out onto the muck.

Firefighting officials said Monday they had identified a bus believed to be filled with bodies and had worked through the night trying to get through the mud.

Flavio Godinho, a spokesman with the civil defense of the state of Minas Gerais, told the G1 news portal that the bus was near the dam that collapsed, but that it was too soon to say how many might be inside.

Rescue efforts were suspended about 10 hours Sunday because of fears that a second mine dam in the southeastern city of Brumadinho was at risk of failing. An estimated 24,000 people were told to get to higher ground, but by afternoon civil engineers said the second dam was no longer at risk.

Areas of water-soaked mud appeared to be drying out, which could help firefighters get to areas previously unreachable. Still, it was slow going for the search teams, and residents were on edge.

“Get out searching!” a woman yelled at firefighters near a refuge set up in the center of Brumadinho. “They could be out there in the bush.”

Brazilian searchers were poised to get reinforcements on Monday, when more than 100 Israeli soldiers and other personnel were set to join the efforts.

Throughout the weekend, there was mounting anger at the giant Vale mining company, which operated the mine, and questions rose about an apparent lack of an alarm system Friday.

Caroline Steifeld said she heard warning sirens Sunday, but there was no alert when the dam collapsed Friday.

“I only heard shouting, people saying to get out. I had to run with my family to get to higher ground, but there was no siren,” she said, adding that a cousin was still unaccounted for.

In an email, Vale told The Associated Press that the area has eight sirens, but “the speed in which the event happened made sounding an alarm impossible” when the dam burst.

People in Brumadinho desperately awaited word on their loved ones. Romeu Zema, the governor of Minas Gerais state, said that by now most recovery efforts would entail pulling out bodies.

The flow of waste reached the nearby community of Vila Ferteco and an occupied Vale administrative office. It buried buildings to their rooftops and an extensive field of the mud cut off roads.

Some residents barely escaped with their lives.

“I saw all the mud coming down the hill, snapping the trees as it descended. It was a tremendous noise,” said a tearful Simone Pedrosa, from the neighborhood of Parque Cachoeira, 5 miles (8 kilometers) from where the dam collapsed.

For many, hope was evaporating.

“I don’t think he is alive,” Joao Bosco said of his cousin Jorge Luis Ferreira, who worked for Vale. “Right now, I can only hope for a miracle.”

The carpet of mining waste also raised fears of widespread environmental contamination and degradation.

According to Vale’s website, the waste is composed mostly of sand and is non-toxic. However, a U.N. report found that the waste from a similar disaster in 2015 “contained high levels of toxic heavy metals.”

Over the weekend, courts froze about $3 billion from Vale assets for state emergency services and told the company to report on how they would help the victims.

Neither the company nor authorities had reported why the dam failed, but Attorney General Raquel Dodge promised to investigate. “Someone is definitely at fault, she said.”

Dodge noted there are 600 mines in Minas Gerais alone that are classified as being at risk of rupture.

Another dam administered by Vale and Australian mining company BHP Billiton collapsed in 2015 in the city of Mariana in Minas Gerais, resulting in 19 deaths and forcing hundreds from their homes.

Considered the worst environmental disaster in Brazilian history, that disaster left 250,000 people without drinking water and killed thousands of fish. An estimated 60 million cubic meters of waste flooded nearby rivers and eventually flowed into the Atlantic Ocean.

Sueli de Oliveira Costa, who hadn’t heard from her husband since Friday, had harsh words for the mining company.

“Vale destroyed Mariana and now they’ve destroyed Brumadinho,” she said.

Other residents quietly noted that Vale was the main employer in the area.

“The company is responsible for a new tragedy, but it’s the principal employer,” said Diego Aparecido, who has missing friends who worked at Vale. “What will happen if it closes?”

Environmental groups and activists said the latest spill underscored the lack of environmental regulation in Brazil, and many promised to fight any further deregulation.

Marina Silva, a former environmental minister and presidential candidate, toured the area Sunday. She said Congress should bear part of the blame for not toughening regulations and enforcement.

“All the warnings have been given. We are repeating history with this tragedy,” she told the AP. “Brazil can’t become a specialist in rescuing victims and consoling widows. Measures need to be taken to avoid prevent this from happening again.”