Washington (CNN) -- Authorities in the District of Columbia responded Friday to several more suspicious letters arriving in the mail at public schools around the city, a day after more than two dozen similar envelopes arrived at city schools.
"This was not unexpected," said D.C. Fire and EMS spokesman Pete Piringer. He told CNN "we thought there might be more that had not yet been processed," either through later delivery or that were not immediately discovered by workers at the schools.
Federal officials as of early Friday said no hazardous substances have been found in the mailings, as detailed testing and investigation continue.
Piringer said all three schools Friday remained open, and that there had been no exposure to any spilled powder that would warrant a hazardous materials response. He said workers would have intercepted the suspicious letters as part of a longstanding mail processing protocol.
"If they find something, they put it aside, they call authorities," he said. "We come and package it up and take it away."
The FBI had said it will look into whether these "powder letters" are linked to an incident in the past which is believed to have involved envelopes sent from a location in Texas.
In a statement, the agency said the letters appear to have been "mailed from the Dallas area and are similar in style and content to other suspicious letters under investigation by Dallas FBI and U.S. Postal Inspectors."
The FBI noted the latest envelopes resemble those received last October at some district schools.
CNN's Terry Frieden, Carol Cratty and Eric Marrapodi contributed to this report