Republican state lawmakers in Tennessee received threatening letters containing a suspicious substance Thursday, prompting an investigation by the FBI, officials said.

The letters were sent to GOP lawmakers' offices inside the Cordell Hull state office building on the grounds of the state Capitol in Nashville, officials said. The FBI is working with law enforcement to review each incident and "safely collect" the letters, which had an "unknown substance," the bureau said in a statement obtained by CNN affiliate WZTV.

"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," the FBI's statement reads.

Tennessee House GOP leaders "received letters containing an unknown substance" and "obvious threats by a liberal activist specifically targeting us," the Tennessee House Republican Caucus said in a tweet Thursday afternoon.

The Nashville Fire Department responded to the scene and used protective suits and air tanks to test the substance on the letter, the agency said in a tweet. No injuries were reported.

Among the factors the FBI is probing are the number of letters sent, the person or people responsible for sending the letters and the motive, the FBI's statement reads.

"As this is an ongoing matter, we will not be commenting further regarding our steps or methods, but the public can be assured that law enforcement will continue to keep the public's safety as its top priority," the FBI added.

Earlier this month, Kansas legislators and public officials across the state received about 100 letters containing a white powder, officials said. That incident is under investigation by the FBI and the Kansas Bureau of Investigation. Officials did not release a motive behind those letters.

As of Thursday, no arrests had been reported in either case.