The letters that contained suspicious white powder and were sent to 6th Congressional District candidate Karen Handel's Roswell neighborhood Thursday appear to be non-hazardous, the FBI said Friday.

Investigators will conduct additional testing to confirm that the powder, sent in more than a dozen letters, was safe, the bureau said in a statement.

RELATED: Suspected Washington shooter posted about Karen Handel

EXCLUSIVE: Expert coverage of 6th District race at MyAJC.com

TV stations FOX 5 and 11 Alive received similar letters.

The letter FOX 5 received was mailed from Greenville, S.C., according to a picture of the envelope provided by the FBI.

A spokesman for the Atlanta office of the FBI declined to comment on whether all of the letters were mailed from Greenville, or whether investigators had any suspects in mind.

Handel’s street was blocked off Thursday afternoon after the “suspicious packages” were discovered at several homes.

In addition to the powder, the letters contained a threatening note that referred to Handel as a “dirty fascist” and encouraged readers to take a “whiff of the powder and join her in the hospital,” according to a copy of a letter that a neighbor provided to The Atlanta Journal-Constitution.

“More than a dozen letters are under review,” the FBI said in the statement. “Of the several letters opened, all contained threat-based content.”

Nine other letters, investigators said, were not delivered because they were intercepted by the U.S. Postal Service.

The incident escalated an already-tense congressional race a day after a gunman opened fire on members of the Republican congressional baseball team in Washington, an attack investigators believe was politically motivated.

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Authorities are still investigating what was in them and who left them in mailboxes.