Operations resumed at Minneapolis-St. Paul International Airport Tuesday morning after smoke was discovered in the control tower and air traffic controllers were evacuated from the building.

Fire crews from the airport's fire department and other agencies arrived at the tower about 5:30 a.m. on a report of smoke in the tower.

Just after 6 a.m., smoke dissipated and "FAA personnel [were] allowed to go back into the facility and resume operations," said Metropolitan Airports Commission spokesman Jeff Lea.

All incoming flights were held at their originating airport until 6:30 a.m., according to the flight tracking website Flightaware.com.

By that time, flights were coming and going and the FAA reported delays of 15 minutes or less for arriving and departing flights.

It was not clear what caused the smoke. Officials continue to investigate.