Two people have died in an accident at one of the stadiums which will be used at the 2014 World Cup in Brazil.
The deaths occurred when a crane fell and destroyed parts of Sao Paulo's Arena Corinthians stadium.
Brazil are due to play the opening match of the tournament at the 65,000 capacity venue on 12 June.
A Fifa spokesman said local authorities would fully investigate the accident and its president Sepp Blatter said he was "deeply saddened".
A statement from football's world governing body read: "We wish to send our heartfelt condolences to the family of the workers who tragically died today.
"The safety of workers is the top priority for Fifa, the LOC [Local Organising Committee] and the federal government.
"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.
"Please understand that we are in no position to comment further at this stage, as we are awaiting further details from the authorities."
Brazilian police said a number of other construction workers were injured in the accident, which happened when the final part of the roof covering the south stand was being installed.
A crane operator was taken to hospital but was later released.
Local media reported witnesses hearing a loud crash as the crane fell on top of a metal structure.
Some workers fled, but most were said to be away on their lunch break at the time of the incident.
Construction company Odebrecht Infrastructure told BBC Sport the structure of the stadium had not been damaged.
A company statement said: "Shortly before 1pm, the crane that hoisted the last module of the structure of the metal roof of the Sao Paulo stadium collapsed causing the fall of the part of the circulation area of the east building - partially hitting the LED facade.
"The structure of the grandstand was not compromised. It was the 38th time this type of procedure had been performed and a number of equal proportions was installed just over a week ago in the southern sector of the stadium.
"Teams of firemen are in place. At the moment, all efforts are focused to provide full assistance to the families of the victims."
BBC Sport's Ben Smith added: "A number of senior figures at Corinthians are now at the scene but they all say this will not prevent them from hosting World Cup matches.
"Before the accident, the stadium was 90% complete with the deadline for completion set for the end of next month. That will now be revised back."
The arena, which will stage five further World Cup games, including a semi-final, in addition to the opening fixture, was due to be completed at the end of December to meet a Fifa deadline.
After the World Cup the stadium - also known as the Itaquerao- is expected to accommodate nearly 50,000 people. However, at Fifa's request, 20,000 temporary seats are to be added ahead of the tournament.
Brazil has admitted it is struggling to have all 12 venues completed on time. Six are not yet ready.
This is the third accident to have caused deaths in World Cup stadiums in Brazil, with two construction workers killed over the past two years.
Corinthians have declared seven days of mourning.