Investigators say a letter with a powdery white substance received by the Dallas County District Clerk's office Tuesday is not related to nearly two dozen other mailings.
Two employees at the clerk's office in the Crowley courthouse reported the letter at about 1:30 p.m. The employees were quarantined for longer than an hour while hazmat teams determined if the substance was toxic.
Authorities said testing showed the substance was not toxic, but was something consistent to a pain reliever.
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Postal inspectors and the FBI said the mailing is not related to 21 other letters with a white powder mailed to locations across the Metroplex since Thursday.
Investigators said an inmate in a Texas Department of Corrections prison sent the letter to the clerk's office.
Thirty employees were evacuated, and dozens of people at the courthouse to pay fines or get documents were unable to do so.
Investigators said they believe a different person mailed 21 other letters -- including one received by the Lockheed Martin facility on South Sherman Street in Dallas on Tuesday morning. One person is believed to be responsible for all of those mailings, investigators said.
The substance in most of those letters was similar to cornstarch, the FBI said.
Federal investigators said they expect additional letters in next several days.
The mailings are violations of federal postal regulations. Each letter could result in a criminal penalty of up to five years in prison. The FBI said the U.S. Attorney's Office could also prosecute the crime under legislation that deals with weapons of mass destruction, which would carry significantly more time prison time.
Anyone with information on who is sending the letters should contact the FBI or local police.
The FBI said anyone who comes into contact with a letter with a powdery substance should immediately wash his or her hands and call 911. Investigators said no one should attempt to touch the substance or dismiss it as a prank because it could be toxic.