Rescuers continued to search rubble in Rio's historic downtown...after 3 buildings collapsed...killing at least 17...with 7 still missing.
Six people were pulled from the rubble.
The commercial buildings were mostly empty Wednesday night.
Officials can't yet fully explain why a 20-story...and a 10-story building...suddenly crumbled onto a 4-story structure.
But illegal construction on the tallest structure may have started a domino effect that resulted in the tragedy.
Others are blaming a gas leaks.