Update (1419 EST):

Two Secret Service agents have been hospitalized after coming into contact with a powdery Ed horse tranquilizer according to reporters.

One suspect is in custody and multiple officers have been medically evacuated after an unknown substance was detected at the Eisenhower Executive Office Building at 17th Street in NW D.C.

The substance has not been identified yet, nor has the suspect. The medical evacuation of officers seems to be routine at this point. This is developing.