Firefighters quickly brought a fire in downtown St. John's under control Monday evening.

Crews arrived at the former CBC Radio building on Duckworth Street just after 6:30 p.m. to find black, thick smoke pouring out of the top floor.

Half a dozen fire trucks responded to the call in an area of the city where many of the buildings are older and built closely together.

Superintendent Don Byrne of the St. John's fire department said two crews of firefighters went into the building initially. He said they traced the fire's origins to the third and fourth floors.

"That seems to be where the heaviest volume of fire was at the time," he told reporters about an hour after firefighters had arrived on the scene.

"There is still some smouldering there, but we have knocked down the fire, and we have it under control. It wasn't so much the flames, it was the volume of smoke with what was burning. And initially we had to do a complete sweep of the building to ensure there were no people inside."

Byrne said firefighters and RNC investigators were back inside the building Monday evening to try to find the cause.

The building has been vacant since the operations of CBC Radio moved out and relocated to its CBC television building nearly five years ago.