(CBS/AP) WASHINGTON — The FBI says the more than 20 suspicious letters delivered to schools in the Washington area are similar to letters mailed to schools elsewhere in the U.S. over the last several weeks.

James McJunkin, the head of the FBI’s Washington field office, says the letters reported Thursday were mailed to about 25 schools in the region.

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Mark White, the FBI’s Dallas spokesman, spoke with KRLD about the incident Thursday afternoon:

[audio http://dfw.cbslocal.com/wp-content/uploads/sites/15909545/2011/05/mark_white_-_dallas_fbi_1.mp3 | Mark White On KRLD]

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He says they were sent from out of state, but declined to be more specific. A television station in Washington obtained an image of one of the letters that had a Dallas postmark.

The District of Columbia letters had a white, powdery substance inside, and McJunkin says they appear to have been sent by the same culprit. He says the previous letters contained white powder that turned out to be harmless.

Pete Piringer, a spokesman for D.C. Fire and EMS, says some of the schools were evacuated for precautionary reasons. He says there have been no illnesses or injuries associated with the envelopes and no one has been hospitalized.

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The first letter was reported Thursday afternoon at Terrell Elementary School in southeast Washington. They’ve been mailed to elementary, middle and high schools around D.C.