Rescue teams intensified search efforts Friday as hopes dimmed for finding survivors of the collapse of three high-rise buildings in central Rio that killed at least 11 and left an estimated 15 missing. Rescue efforts have been under way since late Wednesday when the three office buildings -- of 20, 10, and four stories -- collapsed near the municipal theater on the city\'s Cinelandia square, a historic district bustling by day but nearly deserted at night. Officials said hopes were fading of finding survivors as the death toll rose to 11, with the discovery of seven bodies Friday. Late Thursday, Rio de Janeiro Mayor Eduardo Paes said the death toll stood at four with 22 missing. Authorities expect the search to end sometime between Friday night and Saturday morning \"to minimize the suffering of families (of the victims),\" said Colonel Ronaldo Alcantara, of the Rio fire department. \"I would love to be wrong, but there is little chance\" of finding survivors, said Sergio Simoes, the secretary for civil defense. \"We are racing against time. I have set a 48-hour deadline to wrap up the search,\" he added. Twelve of the missing were believed to be in a classroom on the sixth floor of the 20-story Liberdade building where a computer firm was giving a training course when the edifice crashed down. \"We expect to find a significant number of people there,\" Simoes said. A dense cloud of smoke hung over the rubble from a fire that was still burning, beyond the reach of firefighters, according Alcantara. Some 390 people are taking part in the rescue effort, including a contingent that was sent to Haiti during the devastating earthquake there two years ago. The disaster could have been far more deadly if it had happened during the day, and it raised fresh questions about Brazil\'s preparations to host the 2014 football World Cup and 2016 Summer Olympic Games. In an office of the nearby Municipal Council, anguished relatives of the missing waited for news. One of the victims, 46-year-old Celso Cabral, was buried Friday in a cemetery of the nearby city of Niteroi. The three buildings collapsed almost simultaneously for as yet unknown reasons, though authorities said they did not believe an explosion had occurred, as witnesses initially reported. But investigators were focusing on a structural problem as the likely cause. Witnesses reported that remodeling work had been taking place on two floors of one of the buildings. The work had not been cleared with the Rio Engineering Regional Council (CREA) and was therefore illegal, according to CREA official Luis Antonio Consenza. Both police and CREA have launched an investigation. Brazil has had its share of disasters in recent months, including explosions caused by gas leaks, and fatal accidents in poorly maintained amusement parks or in packed streetcars with faulty brakes. And authorities are scrambling to upgrade the country\'s infrastructure and to build or renovate 12 stadiums around the country in time for the World Cup, one of the world\'s premier sporting events. Last month football\'s ruling body FIFA warned Brazil about delays in the progress of construction projects expected to be ready for the four-yearly extravaganza. The Getulio Vargas Foundation and consultancy Ernst & Young have said the country needs more than $11 billion in investment to fix roads, boost hotel capacity, reinforce security and develop its telecom network ahead of the Cup.