Four county elections offices in Washington state were evacuated Wednesday after they received envelopes containing suspicious powders — including two that field-tested positive for fentanyl — while workers were processing ballots from Tuesday's election.

The elections offices were located in King County — home of Seattle — as well as Skagit, Spokane and Pierce counties, the Secretary of State's Office said in an emailed news release. Local, state and federal agents were investigating, and no one was injured, officials said.

A senior U.S. official familiar with the investigation told CBS News on Thursday that roughly a dozen letters were sent to addresses in California, Georgia, Nevada, Oregon and Washington state. This official was unaware of letters being found anywhere else. 

The substance found on an unspecified number of the letters — not all of them, just some — included traces of fentanyl, the official said, adding that the substance overall was described as "nonharmful." The substance was identified using preliminary field tests, not more rigorous lab tests at FBI facilities, the official said.

Federal investigators believe the letters are being sent from a location in the Pacific Northwest, but the official could not describe the content of the letters, saying the information would have to come from the FBI.

The FBI is conducting its own lab tests, a separate U.S. official familiar with the investigation confirmed to CBS News on Thursday.

Police detective Robert Onishi of Renton, Washington, confirmed that an envelope received by workers at a King County elections office field-tested positive for fentanyl, while Spokane Police Department spokesperson Julie Humphreys said in a news release that fentanyl was found in an envelope at the Spokane County Elections Office. 

The envelope received by the Pierce County elections office in Tacoma contained baking soda, Tacoma police spokesperson William Muse told The Seattle Times.

Similar incidents were reported in other states, with the FBI's Atlanta Bureau saying that it, and other law enforcement partners, had 
"responded to multiple incidents involving suspicious letters sent to ballot counting centers nationwide." The office did not say what other states had received such letters, or provide information about where in Georgia the suspicious envelopes had been received.

Georgia Secretary of State Brad Raffensperger confirmed Fulton County had been targeted. "Our No. 1 priority is secure elections and protecting the men and women who secure our elections," Raffensperger said in response to the reported threats. 

The Department of Justice said it was "aware of the reports" and that the FBI and U.S. Postal Inspection Service were investigating the reports. USPIS declined to comment further. 

A message inside the envelope said "something to the effect of stopping the election," Muse said. "There was no candidate that was identified. There was no religious-affiliated group identified. There was no political issue identified. It was just that vague statement."

Voters in Washington state cast their ballots by mail. Tuesday's elections concerned local and county races and measures, including a question on renter protections in Tacoma, a tight mayor's race in Spokane and close city council races in Seattle.

Secretary of State Steve Hobbs called the incidents "acts of terrorism to threaten our elections."

"These incidents underscore the critical need for stronger protections for all election workers," he said.

Halei Watkins, communications manager for King County Elections, told The Seattle Times the envelope opened by staffers in Renton on Wednesday morning was not a ballot. By 3 p.m., King County had returned to counting and was planning to meet its original 4 p.m. deadline to post results, but the update would be "significantly smaller" than what is usually posted on the day after an election, Watkins said.

Patrick Bell, a spokesperson for Spokane County Elections, said workers were sent home after the envelope was found mid-morning and no further votes would be counted Wednesday.

The FBI warned that all people should exercise care in handling mail, especially from senders they don't recognize. Toxicology and public health experts have previously told CBS News that just touching or being near fentanyl won't cause an overdose.

The Secretary of State's Office noted that elections officials in two counties — King and Okanogan — received suspicious substances in envelopes during the August primary. In the case of King County, the envelope contained trace amounts of fentanyl, while in Okanogan the substance was determined to be unharmful on testing by the United States Postal Inspection Service.

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