The suspicious deliveries began showing up last Friday, when 14 campuses reported receiving envelopes containing an unknown white powder.
"Hazmat came out and did some field-testing and found the powder to be non-hazardous. That needs to be followed-up with lab tests, but the field tests showed non-hazardous."
HISD spokesperson Norm Uhl says all of the envelopes appear to be identical.
"Basically, a common, white letter envelope with a less than a teaspoon of powder inside and nothing else. No letter, no note, nothing inside except for the powder."
Uhl says the suspicious mail has not disrupted classes, nor have the envelopes arrived in any areas of the schools where students might be exposed to them. The U.S. Postal Service is investigating the source of the mail. The district says it can't think of any obvious suspects. Even if this is just a prank, whoever is responsible for the mailings could face fines and/or prison time.