Washington, Jun 23 (IANS): The FBI has launched a probe into an incident of Republican lawmakers in the US state of Tennessee receiving threatening letters containing a suspicious substance, officials said.

In a statement, the FBI said that the letters were sent on Thursday to the lawmakers' offices inside the Cordell Hull state office building on the grounds of the state Capitol in Nashville, reports CNN.

"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 statement says.

Also confirming the incident, the Tennessee House Republican Caucus said in a tweet on Thursday night: "TN House GOP leaders received letters containing an unknown substance. The letters contained obvious threats by a liberal activist specifically targeting us. 6th fl. of CHB remains on lockdown while 1st responders investigate. All employees & members in the building are safe."

According to the FBI statement, the Bureau is probing a number of factors, including the number of letters sent; the person or people responsible for sending the letters; and the motive.

"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," it added.

Earlier this month, Kansas legislators and public officials across the state received about 100 letters containing a white powder, officials said.

The incident is currently under probe by the FBI and the Kansas Bureau of Investigation.