More anthrax hoax letters expected

More anthrax hoax letters are expected this week in the Washington, D.C., area after mail was delivered over the weekend.

At least 33 of the suspicious letters were mailed to public schools in the D.C. area and six others were also discovered last week. The letters all contained white powder and the phrase “Al Aqeda-FBI” typed on a piece of paper, MSNBC reports.

One school showed the substance to be cornstarch, but it was not immediately known if the same substance was in the other letters. The letters appear similar to hundreds of previous letters sent from North Texas to U.S. embassies around the world, at least 40 governors’ offices throughout the country, airports, schools and at least seven United Nations missions in New York City.

No one has been arrested over the more than 250 mailings. While expensive hazmat responses have occurred in every case, the powder has always turned out to be harmless.

"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," Amanda McMurrey, a U.S. Postal Inspection Service spokeswoman said, according to MSNBC. "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."

Investigators have not said if this latest round of letters is believed to be connected to the previous cases.