Denver, Colorado - US Representative Lauren Boebert was among a group of mostly Republican politicians and officials who were sent suspicious letters laced with an "apparently harmless white powder," the Wall Street Journal reported Wednesday.

The FBI is investigating the origin of the letters, a spokesperson confirmed to the Denver Post on Wednesday.

More than 100 letters were sent to various public officials across the country, according to the Journal, including Boebert, conservative US Supreme Court Justice Clarence Thomas, and Republican state lawmakers in Kansas and Montana.

Several letters were intercepted by the US Postal Service and had been discovered in Colorado, Kansas, Florida, Georgia, Washington DC, Montana, and Tennessee.

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Vikki Migoya, spokesperson for the FBI’s Denver office, said in a statement to the Post that some of the letters "contained an unknown substance."

Testing is ongoing to determine the exact substance in the letters but initial results indicated no risk to public safety.

While the Journal reported that many of the letters were sent to elected Republicans, Migoya said investigators are still working to determine an exact motive.

"Law enforcement and public safety officials are working to determine how many letters were sent, the individual or individuals responsible for the letters, and the motive behind the letters," she wrote in an email. She declined to comment further when asked to confirm if letters were sent to Boebert or any other Colorado official.

Messages sent to Boebert’s office were not immediately returned Wednesday afternoon. Colorado spokespeople for the Postal Service said they were gathering more information about the letters.

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According to the Journal, the letters were sent in standard white envelopes with fake return addresses. Some were sent under the names of dead transgender people, the newspaper reported.

Many letters were signed from "your secret despirer," an apparent misspelling. It’s unclear if any of the intended recipients actually received or opened the letters.