Scott Gordon, NBCDFW.com
At least 29 letters with a North Texas postmark containing white powder were mailed to Washington, D.C.-area schools.
The FBI is investigating more than three dozen letters with white powder that were mailed from a North Texas postmark to schools in the Washington, D.C., area.
At least 33 suspicious letters were mailed to public schools in the D.C. area. Six other letters were also discovered. All of the letters contained white powder and the words "Al Aqeda-FBI" typed on a piece of paper.
Special Agent Mark White, a Dallas FBI spokesman, said more letters are likely to surface next week after mail over the weekend is delivered.
The D.C. Emergency Management Office said one school showed the substance to be cornstarch, but it was not immediately known what powder was in the the other letters.
The letters appear similar to to hundreds of previous letters sent from North Texas to at least 40 governors' offices around the country, U.S. embassies around the world, at least seven United Nations missions in New York City, airports and schools since 2008.
Locally, recipients have included Dallas Love Field airport, an aircraft equipment business in Arlington, a church in Richardson and high-tech companies in Dallas and Garland.
No one was has ever been arrested in the more than 250 mailings.
All of the previous cases have prompted expensive hazmat responses, but the powder has turned out to be harmless every time.
"People should feel secure in their mail system because we do have a mail system called biohazard detection system that looks for signs of any sort of biohazard, like anthrax," said Amanda McMurrey, U.S. Postal Inspection Service spokeswoman. "So when mail like this goes through the postal service, it will go through this media system. So when it reaches the other end, most likely, there's absolutely nothing to be afraid of."
So far, no similar letters have been reported to be received in North Texas. Investigators have not said if the latest round of letters are believed to be connected to previous cases.
NBC DFW's Scott Gordon, Ken Kalthoff and Randy McIlwain contributed to this report.