Authorities are continuing to investigate more than a dozen envelopes containing suspicious white powder that were sent to Houston Independent School District campuses on Friday.

Preliminary tests of the substance, conducted on each envelope sent to all of the 14 schools that received the envelopes, showed it to not be dangerous, officials said. No injuries were reported.

At this time, all incoming mail sent to all HISD campuses is being inspected. The district will not receive any mail until Tuesday because of the Columbus Day holiday.

Along with HISD police, the FBI and the U.S. Postal Inspection Service are investigating.

The incident began about 1:30 p.m. Friday when staffers at Attucks Middle School and Anderson Elementary told HISD officials that they had received the envelopes.

"It was a typewritten envelope with powder — no note — addressed to the school," said Sam Sarabia, head of HISD elementary schools.

HISD officials then asked the other schools to check through their afternoon mail. Each school that received a powder-filled envelope followed the same protocol: Isolate it and report the discovery, Sarabia said.

Each envelope contained less than a teaspoon of the powdery substance. The initial field testing indicated no signs of radiation, explosives or volatile chemicals, HFD officials said.

Some of the schools had already released their students when the envelopes were found. HISD officials said the incident, while serious, had little impact on overall operations at any of the campuses.

"The mail comes through the administrative section of the building. A student would not have any contact with the mail," HISD Police Chief Jimmie Dotson said.

Parents of students in the affected schools were notified about the envelopes by a telephone alert system, officials said.

Classes will resume as scheduled on Monday.

By late Friday, authorities had received no calls from anyone attempting to claim responsibility for the substance, officials said.

"We will certainly get with the (Harris County district attorney's) office to see if there are any charges that can be filed if the person is identified," Dotson said.

Even if the incident turns out to be some kind of prank, HISD officials said they worry that it may spawn copycats.

"We're hoping that this is the end of it. We'll be prepared if it's not," Dotson said.