Felipe Dana / AP

Rescue workers search for victims Wednesday after a building collapses in Rio de Janeiro, Brazil.

Updated at 3:40 p.m. EST Thursday:

RIO DE JANEIRO -- Rescuers pulled at least six bodies from the rubble of three buildings that suddenly collapsed, the Rio de Janeiro morgue said Thursday.

At least 16 people remained missing as smoke from small fires drifted above the wreckage, The Associated Press reported.

Authorities speculated that illegal construction work damaged the structure of a 20-story building and caused it to crumble, bringing down two other office buildings alongside at about 8:30 p.m. Wednesday.

Rescue crews, aided by specially trained dogs, dug through tons of brick and twisted metal.

"We have hopes of finding people alive," said Moises Torres, a spokesman with the Fire Department.

According to the institution responsible for approving construction in Rio, unauthorized construction projects were under way in the building. The head of the accident prevention unit of Rio's Regional Council of Engineering, Luiz Cosenza, told the Globo television network that illegal projects could have led to the collapse.

"Two projects were happening in the building," said Cosenza. "They were illegal works; they were not registered with the council."

He didn't provide details but said the work was not being supervised by any registered professional.

Hoping for news
A janitor finishing up the day's work was among six injured extracted by rescue crews from the heap of bricks, metal and glass. An accountant closing the books for the month and a computer technician installing software were among the missing. Their friends or relatives took shelter from the scorching sun in a nearby building, hoping for news.

Francisco Adir was trying to get information about a friend, Flavio Porrozi, 23, who had been attending a computer course in the largest of the three buildings.

"We think he's alive. At 3 a.m. he managed to call his girlfriend and say, 'Hello, love,' before his phone went dead," Adir said. "The rescuers haven't given us any information, but the family is hanging all their hopes on that phone call."

Five of the six bodies pulled out have been identified in the city morgue, and Porrozi is not among them.

As the hours ticked by, relatives of the missing tried to keep despair at bay.

"We last heard from him at 8 p.m. when he called his wife to say he wouldn't be much longer," said Luis Cesar Vasconcelos, whose brother, computer technician Luis Leandro Vasconcelos, remained trapped in the debris. "Since then, there's been no sign of him, but the family is hopeful to the end."

The state's governor, Sergio Cabral, issued a statement saying the government was doing all it could to support the families of the victims.

"We're still living a moment of shock," he said. "There is still hope of finding survivors, and in a last instance, of rescuing bodies."

Updated at 1:40 p.m. EST Thursday:

RIO DE JANEIRO -- Three bodies were pulled from the rubble of three collapsed buildings on Thursday, an official with the Rio de Janeiro Fire Department said. Another 21 people were still missing after the buildings went down in the city's historic center, The Associated Press reported.

Mayor Eduardo Paes said a structural problem may have caused a building of about 20 stories to collapse at about 8:30 p.m., and that apparently caused the collapse of two smaller buildings nearby. Officials were still investigating the causes, however.

In addition to the dead and missing, at least six other people were treated for injuries caused by the accident, which left rubble and thick dust strewn over a wide area near Rio's famed Teatro Municipal and the Fine Arts Museum.

One of those pulled out alive was Marcelo Moreira, a janitor in an eight-story building that fell.

"He stayed behind to finish a little bit of work," said Rosalvo Alves, the building's main doorman, who had spent the night in a local hospital with his friend. "We shut down at 8. I left, and he was supposed to come too. Now this; he's hurt, our jobs are gone, everything is gone."

Alves worked in the building for 38 years, and said he had never noticed any problems.

Original post:

RIO DE JANEIRO -- Two downtown buildings collapsed Wednesday, leaving at least two dead inside the wreckage.

Rescue crews pulled four people alive from the debris, officials said.

One building was 20 stories tall; the other, 10, Reuters reported.

A loud explosion preceded the collapse, witnesses said.

There was a strong smell of gas in the area, officials said. However, a gas leak likely didn't cause the blast, Rio's mayor said. 

Rubble was strewn across a wide area, covering cars and motorcycles, The Associated Press reported. Television showed at least two people on the roof of a neighboring building where they apparently awaited help from firefighters.

"It was like an earthquake. First some pieces of the buildings started to fall down. People started to run. And then it all fell down at once," a witness who identified himself as Gilbert told Reuters.

As many as 11 people could be trapped in the wreckage, cable broadcaster Gloominess said. City authorities are assessing the risk of collapse of a damaged adjoining building, local news services reported, without giving the source of the information.

"I ran down the stairs desperate to escape. Just when I left the building it collapsed. I escaped by the skin of my teeth - it's the work of God," Nelson Tomes, 38, an air conditioning technician, told iG news service.

Tomes, who was on the 10th floor of one of the buildings, said he rushed to the stairwell after he heard a "huge noise."

TV images showed cars covered with concrete and steel rods. Light, the electricity distribution company serving the city, cut power to the area to avert the possibility of fires after the gas smell was detected, the TV broadcasters said.

Rio de Janeiro is struggling to address concerns over poor infrastructure as it prepares to co-host the soccer World Cup in 2014 and the Olympic Games two years later.

The incident comes months after an explosion apparently caused by a gas leak ripped through a restaurant in downtown Rio, killing three people and igniting concerns over the state of the city's infrastructure.

Vanderlei Almeida / AFP - Getty Images

Firefighters look for victims amid the rubble of a building that collapsed Wednesday in downtown Rio de Janeiro, Brazil.

Reuters and The Associated Press contributed to this report.