Montana legislators are the latest lawmakers to receive "anonymous, threatening letters containing white powder," amid an FBI investigation into similar incidents across the country, according to state officials.

On Thursday, Republican state lawmakers in Tennessee received threatening letters containing a suspicious substance. 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 said the letters had an "unknown substance."

Last week, about 100 letters containing a white powder were sent to public officials across Kansas.

"At least two Montana legislators have received suspicious packages in the mail containing a white powder. At this time, both are fine and law enforcement is investigating and will be testing the substance," the state Senate GOP tweeted.

Montana Gov. Greg Gianforte on Friday said he's "received disturbing reports that Montana legislators are receiving anonymous, threatening letters containing white powder. The state will bring to bear whatever resources are needed to support law enforcement officers as they investigate."