The FBI and U.S. Postal Inspection Service are investigating a series of letters sent to election offices containing white powder. Some state officials called the incidents "acts of terrorism."

One envelope included powder that tested positive for fentanyl, law enforcement officials told NBC News on Thursday. No one has reported suffering ill health effects, the officials said, but the substances are still being tested by labs.

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Officials in California, Georgia, Nevada, Oregon, and Washington state have all reported instances of suspicious letters addressed to election officers or workers over the past few days.

The FBI in Portland said it "responded to multiple incidents involving suspicious letters sent to several ballot counting centers in Oregon." Seattle and Atlanta FBI field offices issued similar statements, according to NBC News.

The Texas Department of Public Safety issued a statement announcing that a letter containing an unknown substance was mailed to the state attorney general's office in Austin on Thursday morning. The department said initial test results returned negative, but the FBI said it would continue testing.

California Secretary of State Shirley Weber said federal and state authorities were investigating suspicious letters after two were intercepted by the U.S. Postal Service addressed to election facilities in Los Angeles and Sacramento.

Georgia Secretary of State Brad Raffensperger announced on Thursday that his office was responding to reported threats to elections officials.

"Our No. 1 priority is secure elections and protecting the men and women who secure our elections," Raffensperger said, confirming that letters were targeting Fulton County.

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“We’re working with our state and federal partners to determine if any additional Georgia officials are being targeted. Domestic terrorists will not trample on our right to free and fair elections," Raffensperger said. "Election officials should be free from fear and intimidation, which is why I’ve called on the General Assembly to increase penalties for election interference. We will work tirelessly to ensure that Georgia elections remain free, fair, and secure.”

Washington Secretary of State Steve Hobbs told the Associated Press the incidents in his state were “acts of terrorism to threaten our elections.”