Story highlights Fire officials say at least 4 are injured; CNN affiliate says 11 are missing

Witnesses say buildings in the area swayed

"When I looked up, I saw the top floors falling," a witness says

Agencia Brasil: Police are isolating the area, searching for victims

Two adjacent buildings collapsed Wednesday night in the historic center of Rio de Janeiro, injuring at least four people, Mayor Eduardo Paes told reporters.

It was not immediately clear what caused the collapses -- one a 20-story building and the other 10 stories -- nor was the extent of the casualties clear.

Paes did not rule out the possibility of a gas leak, but other hypotheses included the possibility of structural failures in the buildings, which were located behind the Municipal Theater.

Fire officials said at least four people were injured, according to a post on the city government's Twitter account. CNN affiliate Band News reported 11 people were missing in the collapses, which occurred before 9 p.m.

"I started to hear a crackling. I thought they were gunshots. When I looked up, I saw the top floors falling," maintenance worker Julio Cesar de Oliveira Brandao told CNN affiliate TV Record.

A fire broke out after the blast, Agencia Brasil said. Police and fire officials were isolating the area and searching for victims, the news agency reported.

Cars parked on the street near the buildings were covered in dust, and there was a strong smell of gas in the area, fire officials said, according to the news agency. The lobby of one of the buildings contained a bank branch and a bakery, Agencia Brasil said.

Witnesses told Band News that neighboring buildings swayed, as if there had been an earthquake. One witness said many people were inside one of the buildings when it collapsed.

The buildings are in a bustling commercial and financial area of Rio de Janeiro.

Electrical engineer Jorge Drauss told TV Record he was drinking juice at a nearby diner when he saw small stones falling from the top of one of the buildings.

"I thought it was someone making a hole for air conditioning," he said. "From one moment to another, the building began to fall."