U.S. Rep. Lauren Boebert is among numerous public officials who have recently been sent suspicious letters containing an unidentified but apparently harmless “white powder,” the Wall Street Journal reported Wednesday.

Authorities have been investigating similar letters sent to lawmakers and other public officials characterized as predominantly Republican for at least a week, with reports of letters received in Kansas, Tennessee and Montana.

The FBI is investigating to identify the source of the letters, a bureau spokeswoman confirmed to Colorado Politics on Wednesday.

The Journal reported that suspicious letters – described as arriving in “standard, white envelopes with fake return addresses” – have also been found in Florida, Georgia and Washington, D.C. At least one was addressed to Supreme Court Justice Clarence Thomas.

More than 100 of the letters have been sent to officials in at least seven states, the Journal reported, with some including the return addresses of dead transgender people and at least listing the return address of a church.

Boebert’s office didn’t respond to a request for comment.

“Some of the letters contained an unknown substance and we are working closely with our law enforcement partners to respond to each incident and safely collect the letters,” Vikki Migoya, spokeswoman for the FBI’s Denver field office, said in an emailed statement. “Laboratory testing is ongoing but at this time has not indicated a risk to public safety. Additional testing will be needed to fully identify the unknown substance in the letters.”

Migoya added that investigators are attempting to determine a motive for the mailings.

“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 said.

Describing the investigation as “an ongoing matter,” she declined to comment further but advised people to “exercise care in handling mail, especially from unrecognized senders.”