CTU Seattle was a Domestic Unit of the Counter Terrorism Unit. The unit was based Seattle, Washington.
Before Day 3
Morris O'Brian worked as an Analyst at CTU Seatle before moving to Los Angeles. Presumably this took place sometime before Day 3, since by Day 3, Chloe O'Brian was working at CTU Los Angeles and was divorced from Morris.
Day 3
During Day 3, CTU Seattle secured a vial of the Cordilla Virus.
It is assumed that CTU Seattle found the vial of the virus but no-one actually said that a CTU branch found it. Seattle is quite a large city so it is assumed they would have a Domestic Unit.
Before Day 4
Michelle Dessler and Bill Buchanan worked together at CTU Seattle.
Before Day 6
Abu Fayed was detained here 14 months prior to Day 6. He was released by Bill Buchanan.