More than 20 letters containing a suspicious white powder with a Dallas-area postmark were sent to public schools in Washington, D.C.

After testing the contents in the envelopes, they don't appear to contain hazardous materials, said FBI special agent Mark White.

The letters were sent from a regional postal center in Dallas, White said. White could not comment specifically on the nature of the substance inside the letters, nor could he specify the exact address used on the letters.

FBI agents at the Dallas office became aware of the situation early Thursday afternoon, White said. More letters are expected to be discovered in the next few days through the beginning of next week, he said.

"It's very common for letters to sit on a desk over the weekend only for the person to discover it the next week," White said.

The letters are similar, but perhaps not identical, to letters sent in 2008 and 2010 to U.S. embassies, governors' offices, churches, mosques, and aeronautical and technical businesses in Texas, Illinois and Massachusetts, a federal official said.

Once the material inside the letters has been analyzed, forensic specialists will likely begin examining the letters for fingerprint, chemical, handwriting and other clues that might be present, said Danny Defenbaugh, a former Dallas FBI special agent who retired from the bureau in 2002.

Behavioral analysis specialists may be able to generate a profile of the person who sent the letters depending on how much verbiage is included, Defenbaugh said. This batch of letters may also point the bureau to additional suspects or help exclude suspects who were identified in earlier cases, Defenbaugh said.

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