Police say they are questioning an individual over "suspicious envelopes". Two letters - one addressed to Barack Obama and the other addressed to Senator Roger Wicker - have been found to contain the poison ricin.
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Guillier conceded a "severe defeat" defeat to his supporters as the result came in.
The outage struck Hartfield-Jackson Atlanta International Airport at around 1pm local time, stopping all outgoing flights.
It will be a chilly day for many parts with the highest temperatures, about 10 Celsius (50F) in the far northwest.