RIO DE JANEIRO — A dam from an open-pit mining complex burst on Thursday in southeastern Brazil, flooding a nearby community and enveloping homes and cars in sludge. Rescue teams descending from helicopters scoured the site for survivors.

The authorities said Thursday night that they were trying to determine the number of casualties, with Brazilian news organizations reporting that at least one person had died. Union officials representing workers at the mine said they feared that as many as 15 people might have died after the dam burst.

The episode in a district of Mariana, a city of 58,000 residents in Minas Gerais State, stunned a country that relies heavily on dams to produce electricity and on mining to generate export revenue. The dam operated by Samarco, a venture between BHP Billiton, the Australian commodities giant, and Vale, the Brazilian mining company, held residue from an open-pit iron ore mining operation.

“We need rigor in determining what happened,” Carlos Eduardo Ferreira Pinto, a prosecutor in Minas Gerais, told reporters on Thursday. “No dam bursts by chance.”