NASHVILLE (AP) — A Tennessee legislative office building in Nashville reopened Thursday after a temporary lockdown prompted by multiple Republican leaders receiving what House Speaker Cameron Sexton called "a white powder substance" in the mail.

Hazmat teams were called to assess the situation, Sexton said in a statement.

The Federal Bureau of Investigation said the letters contained an "unknown substance." "Laboratory testing is ongoing but at this time has not indicated a risk to public safety," the FBI said. "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."

The incident comes a few days after officials in Kansas said that about 100 letters containing suspicious white powder, which preliminary tests found negative for common dangerous toxins, were sent to lawmakers at their homes and to other public officials across the state. Authorities did not immediately say if they believe the Tennessee and Kansas events are connected.

A series of emails to the Tennessee legislative building staff Thursday began with notice that no was allowed on the 6th floor. About an hour later, another said no one could leave or enter the building and an hour after that all but the sixth floor reopened. Access was completely reinstated shortly thereafter, said House Republican Caucus spokesperson Jennifer Easton.

She said the letters "contained obvious threats made by a liberal activist specifically targeting Republicans." She didn't provide further details.

The locked down Cordell Hull building is located in downtown Nashville, where it houses lawmakers and legislative staffers, as well as the General Assembly's committee hearing rooms. The sixth floor is mostly compromised of House offices for both lawmakers and their staff.

The building connects to the Tennessee Capitol's main building. While lawmakers and staffers work there all year, noticeably fewer members of the public are present now, when the General Assembly is out of session.

The GOP-dominated Legislature is preparing for an Aug. 21 special session called by Republican Gov. Bill Lee, who is pushing to remove firearms from people judged dangerous to themselves or others.

The proposal gained public momentum in the wake of a Nashville school shooting in March that killed three young children and three adults. Republican lawmakers declined to take it up in April at the end of the regular annual legislative session, and some continue to say they can't support it.

Tennessee's Legislature garnered national attention earlier this year when Republicans expelled two Democratic lawmakers for leading a protest for gun control from the House floor days after the Nashville school shooting.

The two were removed from office and later reinstated. A third Democratic lawmaker was spared expulsion by one vote.