5:18 PM: Seattle Fire and Police are investigating an “unknown substance” (described on emergency radio as a “white powdeer”) on board a bus downtown. Here’s the most-recent Metro alert:

Buses are currently being rerouted off 3rd Av S between S Washington St and S Main St due to a hazmat response.

Routes 7, 14 & 36 – Use the stops southbound on 3rd Av S just north of James St or eastbound on S Jackson St just west of 5th Av S;
Routes 21 & 116 – Use the stops southbound on 3rd Av S just north of Columbia St or southbound on 1st Av S just south of S Atlantic St;
Route 124 – Use the stops southbound on 3rd Av just north of Columbia St or southbound on 6th Av S just south of S Royal Brougham Wy;
Routes 131 & 132 – Use the stops southbound on 3rd Av just north of Columbia St or southbound on 4th Av S just north of Edgar Martinez Dr S.

5:37 PM: Metro says buses are back to normal routing; SFD “determined it was a non-hazardous substance.”