(CNN) -- Thirteen schools in the Houston, Texas, area have received suspicious envelopes with white powder inside, though the substance does not appear to be hazardous, a spokesman for the school district said Friday.

The substance does not contain toxic, biological or radiological contaminants, according to field tests by hazardous materials teams, Houston Independent School District spokesman Norm Uhl said.

The envelopes each contained less than a teaspoon of white powder and were addressed to the schools without any note inside, Uhl said.

The Houston Health Department and other local agencies are running lab tests on the substance, Uhl said.

After school officials got calls from two schools that received envelopes with the powder inside, they sent word to the district's 298 schools "to very carefully check their mail," Uhl said.

Students were in school at the time the envelopes were discovered and were dismissed at normal time.

The Houston Fire Department is leading an investigation, Uhl said, assisted by the Houston Independent School District Police and Houston Police.

The schools that received envelopes are Alcott Elementary, Almeda Elementary, Anderson Elementary, Ashford Elementary, Attucks Middle School, Barrick Elementary, Bastian Elementary, Black Middle School, Blackshear Elementary, Browning Elementary, Durkee Elementary, Fonville Middle and Bellaire High School, Uhl said.