(CNN) -- Letters containing an unidentified white powder arrived with the morning mail at three more Houston, Texas, schools Monday, bringing to 17 the number of campuses that have received such suspicious packages in the past week, a school spokesman said.
Fourteen letters received Friday all tested negative for toxic substances, but principals at all 298 Houston public schools were put on alert to "to very carefully check their mail," Houston Independent School District spokesman Norm Uhl said.
There was no interruption for students when the latest letters were found at Austin High School, Askew Elementary and Lewis Elementary on Monday morning, Uhl said.
"They may have noticed the fire trucks from class," he said.
The substances tested Friday did not contain toxic biological or radiological contaminants, according to field tests by hazardous materials teams, Uhl said. The envelopes each contained less than a teaspoon of white powder and were addressed to the schools without any note inside, he said.
The schools that received envelopes last week were Alcott Elementary School, Almeda Elementary, Anderson Elementary, Ashford Elementary, Attucks Middle School, Barrick Elementary, Bastian Elementary, Bellaire High School, Black Middle School, Blackshear Elementary, Briarmeadow Elementary, Browning Elementary, Burbank Middle and Fonville Middle, Uhl said.
CNN's Dave Alsup contributed to this report.