Ten more suspicious letters were found in the nation's capital Friday, bringing the total number to 39 in the last two days, the FBI said.

All 39 letters appear to have been mailed from the Dallas area and have the same characteristics as letters under investigation by FBI officials in Dallas and postal inspectors there. Authorities were bracing for even more letters to surface throughout the day.

Authorities in Washington are telling their counterparts across the country to be on the lookout for letters with Al Qaeda-related wording and a suspicious white powdery substance in them, after dozens of schools in Washington began receiving such letters on Thursday.

Federal and state authorities issued a "bulletin" to hazardous materials units across the country, informing them of the latest developments.

The FBI's Washington Field Office said in a news release Friday that no hazardous substances have been found in any of the mailings.

FBI spokesman Lindsay Godwin said agents collected letters Friday morning from four additional schools. She also said four more letters were collected at a mail facility Friday morning by U.S. postal inspectors. She did not know which schools those letters were addressed to.

Authorities said they do not believe the references to Al Qaeda have any particular significance, saying they are likely just an attempt to get attention.

The letters and all other evidence collected will be sent to the FBI's lab at Quantico, Va., for further analysis.

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Fox News' Mike Levine and The Associated Press contributed to this report.