Two letters were opened Tuesday by a machine in the mailroom at Elgin's Chase Bank Credit Card Processing Center. It's a building not open to the public, but seven employees were in the room at the time.
The incident brought hazmat crews from around the northern suburbs, and the powder was found to be harmless white chalk, similar to that used by gymnasts.
More than 30 similar letters have turned up in nine cities around the country, including Chicago, Cincinnati, Dallas, Denver, Newark, New York, Oklahoma City and Washington.
The letters threatening the Elgin location were mailed from Amarillo, Texas, on Saturday, but the person who sent them remains a mystery.