Air traffic controllers were forced to evacuate their tower at Dulles International Airport on Wednesday when fumes from a nearby worksite began to enter the ventilation system at the facility.

Controllers were moved to a backup tower and able to resume near normal operations until the source of the smoke was located and the facility ventilated.  The source of the problem was unauthorized welding by work crews at an adjacent terminal building.

FAA spokeswoman Laura Brown said air traffic controllers were moved to another tower that is normally used to coordinate planes at airport gates. Both towers have the same capabilities, though the backup tower has fewer work positions, she said.

The entire incident lasted approximately two hours and controllers were allowed to return to the main control tower shortly after lunch.