The FBI now says it is investigating at least 20 suspicious letters with white powder that have been delivered to schools in Washington.
Christopher Combs, who oversees domestic terrorism investigations for the FBI's Washington field office, tells WRC-TV that more letters could still be found.
Authorities say there are no indications that the letters contained any hazardous material, and there are no reported injuries or illnesses. Combs says they are being taken to a lab where the material inside will be examined.
He says the FBI has an open case related to similar letters in another jurisdiction, though he didn't say which one. He says none of those letters contained harmful material.
The first letters in D.C. began popping up at schools Thursday.