Brazil has been plagued by a series of setbacks including cost overruns, stadium delays, accidents, labor strife and massive street protests in the run-up to the June tournament, once envisioned as a coming out party for South America's largest nation, which is also scheduled to host the Olympics in 2016.
Wednesday's accident at Sao Paulo's Itaquerao stadium occurred when a construction crane crashed into a 500-ton metal structure that in turn cut through the outer walls of the venue, destroying rows of seats and slamming into a massive LED panel that runs across the stadium's facade.
The accident could delay delivery of the stadium, which was practically finished before Wednesday's collapse. FIFA has set a December deadline for all 12 World Cup venues to be ready. The World Cup begins on June 12 with Brazil playing at the Sao Paulo stadium.
Officials said there were no major structural damages to the stadium but declined to say how much the accident may affect the delivery.
"I don't want to know about FIFA right now, we are worried about the families of the victims," former Corinthians president Andres Sanchez told a news conference.
Sanchez said two people were confirmed dead and nobody else was injured. A fire department official had said earlier that three people had died.
One of the workers, 42-year-old Fabio Luis Pereira, was inside a truck that was hit by the metal structure. The other, 44-year-old Ronaldo Oliveira dos Santos, was resting in an area which was supposed to be isolated.
"Unfortunately nobody saw him and he himself knew that he was not supposed to be there," Sanchez said. "He was napping and unfortunately there wasn't time for him to get out."
The accident happened at lunchtime, so not many of the nearly 1,700 employees working on the venue were on site when the crane collapsed on top of the metal structure, causing the deadly domino effect.
"The sound was as loud as a thunderclap or a huge explosion," said Rodrigo Vessoni, a reporter with sports daily Lance who said he had just walked out of the stadium after interviewing Sanchez. "There was a lot of running around, a lot of shouting. It was frightening. Chills ran through my entire body. It was unbelievable. The noise was metal grating on metal. It was a terrible thing to see."
An official with constructor Odebrecht said a similar metal structure had already been installed with the same crane at the other side of the venue earlier this year.
"Everything was being done according to procedure," said Frederico Barbosa, the construction site's manager. "We will have to wait for the investigation to find out what caused the crane to collapse."
Sanchez reiterated that it appeared to engineers on site that the structure of the stadium had not been compromised, meaning there could be enough time to get everything fixed in time for the World Cup. He said that around 30 percent of the stadium will be closed off pending authorities' investigations.
"Structurally very little was affected," he said.
Brazil is running against time to deliver the last six World Cup stadiums by the end of the year, although work at the Itaquerao was advanced compared to the other venues. FIFA has been pressuring local organizers to make sure all venues are ready by the December deadline so all test events can take place in time for the World Cup.
FIFA said it would not accept the same delays that plagued stadium construction before the Confederations Cup, when only two venues were delivered in time.
The Sao Paulo stadium, which cost nearly $360 million, will seat nearly 70,000 people for the opener. About 20,000 seats will be temporary and installed only for the World Cup.
The venue is scheduled to host another five matches, including a semifinal. It was initially expected to be built for the Confederations Cup this year, but delays with financing for the venue prompted authorities to scrape the project from the World Cup warm-up tournament.
"FIFA and the LOC have learnt of the death of workers at the Corinthian's Arena site in Sao Paulo with great sadness," FIFA said in a statement. "We wish to send our heartfelt condolences to the family of the workers who tragically died today."
Soccer's governing body said the "safety of workers is the top priority" to World Cup organizers.
"We know the safety of all workers has always been paramount for all the construction companies contracted to build the 12 FIFA World Cup stadiums," it said in a statement. "The local authorities will fully investigate the reasons behind such a tragic accident."
FIFA President Sepp Blatter said he was "deeply saddened by the tragic death of workers" at the Corinthians stadium. "Our heartfelt condolences are with the families," he said.
FIFA Secretary General Jerome Valcke said he was "extremely shocked by the news from Sao Paulo."
"Our thoughts are with the families of the victims of this accident," he said.
It wasn't the first problem with World Cup stadiums in Brazil.
One worker died during construction of a stadium in the capital Brasilia last year and another in the Manaus venue in March. Also in March, heavy rains flooded the construction site of the Maracana Stadium, forcing the cancellation of a FIFA inspection visit at the time. In May, a small part of the roof at the Salvador stadium fell in after it wasn't able to sustain the large amount of water that settled on top of it.
In April, another worker died at the construction site of the new Palmeiras stadium, which may be used for teams training for the World Cup in Sao Paulo. That construction site was stopped for 10 days then, when damage was not nearly as significant as it was at the Corinthians venue.
In Rio, the stadium which will host athletics in the 2016 Olympics has been closed for several months because of fears that its roof could collapse. Renovation work is underway at the venue.
Associated Press writers Stephen Wade and Jenny Barchfield contributed from Rio de Janeiro.