SAO PAULO — Two workers were killed on Wednesday when a crane collapsed at the stadium staging the opening match of next year’s World Cup of soccer, causing damage to the exterior and renewing questions about whether Brazil will be ready to host the event on time.

Construction workers had been racing to finish the stadium in Sao Paulo, Brazil’s biggest city, prior to a December deadline imposed by world soccer body FIFA.

Andres Sanchez, a former president for the Corinthians soccer club who is helping oversee construction, told reporters that a crane collapsed while lifting a piece of the stadium’s roof into place, sending the piece tumbling down.

He said the cause of the accident was still unclear and said any possible delay to construction was “the least of our worries.” The union representing the workers said it expected construction would be frozen for up to 30 days while authorities investigate.

Two workers at the site were killed, Sanchez said, with no additional casualties. Emergency officials had previously given conflicting information on the death toll and possible injuries.

The incident casts yet another shadow over Brazil’s preparations for the World Cup, which have been plagued by delays, accidents, cost overruns, and public anger over government waste that contributed to massive nationwide street protests last year.

Workers have now been killed at three World Cup stadiums — in Sao Paulo, Brasilia, and Manaus. Officials have expressed worries about facilities in Manaus and Cuiabá being ready by a December deadline, and Wednesday’s accident seems likely to delay Sao Paulo as well, though it’s unclear for how long.

The São Paulo stadium, formally called Arena Corinthians but known locally as Itaquerão for the area where it is located, was 94 percent complete at the time of the accident, according to the stadium’s website. The facility is due to host the Cup’s opening match on June 12 next year.

Sanchez said the accident did not cause structural damage to the stadium’s stands, which likely would have caused the most extensive delays to construction. A statement by Corinthians, which will play in the facility after the cup, said the piece of roof fell on top of the concourse area on the eastern side of the stadium, “partially hitting the facade.”

Photos and TV images from the scene showed a huge piece of white scaffolding smashed through the exterior, collapsing the top half in the area where it hit.

It was not clear if the damage would pose a major delay to opening the stadium, according to Ricardo Trade, executive director of the local organizing committee for the World Cup.

“It’s impossible to make any forecasts at this moment,” Trade said. “What we know is what the images are showing, that part of the structure fell down.”

“At the same time, we can’t think that if [construction] is delayed by three months, the stadium will end up out of the World Cup.”

Frederico Barbosa, operational manager at the site for Odebrecht SA, the industrial conglomerate building the stadium, said rain had delayed the placement of the roof in recent days. There was no precipitation in the city on Wednesday.

“Conditions were ideal,” he said. “Now we have to find out what happened.”

The state prosecutor’s office said in a statement that it would begin an investigation but it would request a suspension of construction “only if technical elements indicate it is necessary.”

Condolences poured in from FIFA and other authorities.

“Extremely shocked by the news from Sao Paulo,” FIFA’s executive secretary Jerome Valcke wrote on his Twitter account. “Our thoughts are with the families of the victims of this accident.”

“We are currently awaiting further details from the authorities, who are investigating this tragic accident,” Valcke added.

Delays surrounding the World Cup have extended well beyond stadiums. Numerous public transportation projects have been canceled or shelved, and workers have toiled 24 hours a day, seven days a week to finish a new terminal at Sao Paulo’s international airport on time.

Six stadiums were used in the Confederations Cup warm-up tournament in June, but several of them were delivered later than FIFA wanted.

Another six, including Itaquerao, are due to be delivered by the end of December but even then some work will remain. Itaquerao, which was due to be finished on schedule, did not plan to install 20,000 temporary seats until February.

The accident also casts further scrutiny on Brazil’s building standards after a death and injuries in construction projects earlier this year. One stadium also faces an 18-month closure to repair flaws in the roof.

The Itaquerao stadium, on the gritty east side of Sao Paulo, was originally budgeted at $152.2 million but that shot up to over 1 billion reais after local authorities decided to hold the prestigious opening match there.

The accident capped a tough day for Sao Paulo, which earlier on Wednesday lost its bid to host the 2020 World Expo. Dubai was the winner.