Preliminary testing determined the powder is corn starch and today, the work continues to try and pinpoint the source.
The envelopes were found at 14 different campuses scattered all over the city on Friday, and investigators are trying to figure out where they came from. Three more schools were added to the list by HISD Monday morning.The schools that received the letters are:
Attucks Middle School
Fonville Middle School
Burbank Middle School
Austin High School
Bellaire High School
The envelopes discovered Monday are just like the ones found on Friday.
"It's just a standard letter sized with the school's name and address type written on it," said HISD spokesperson Norm Uhl.
The total number of schools targeted by the mystery mailer now stands at 17, most of them beginning with the letter "A" or "B," with the exception of one -- Fonnville Middle School. A suspicious envelope found at Lewis Elementary School Monday morning was actually addressed to Bellfort Academy next door.
Abraham Rendon and his father came to check on Abraham's third-grade brother at Lewis. Abraham says the schools told him and his father where the letters were postmarked.
"They bring a powder to school, it's from Dallas," he said. "That happened to all kinds of schools."
The US postal inspection service would not comment on whether the letters were indeed postmarked Dallas. In a statement to the media, however, a spokeswoman wrote, "US postal inspectors have implemented a process with the U. S. Postal service and HISD officials to capture additional (suspicious) envelopes prior to delivery to the schools."
At the same time, test results for the suspicious powder in eight of the envelopes came back.
"The tests we conducted on the envelopes Friday came back as negative for any harmful biological agent," said Kathy Barton with the City of Houston Health Department.
Because the mailing of the envelopes is a terroristic threat, the FBI is taking the lead and HISD is trying to reassure parents, even as the district helps in the investigation.
"We've actually asked the schools to double check and we haven't heard from any additional schools," said Uhl. p>
Further testing of the powder will take about two weeks to complete, and if the mailer is caught, he or she faces federal charges, to be determined by the US Attorney's office.