Updated: 5:00 p.m. ET 

Tennessee state officials were responding to a rash of bomb threats called in to county courthouses and government offices in 29 counties on Tuesday, the Tennessee Emergency Management Agency said.

The calls — many directed to county clerks — started coming in about 10:15 a.m. CT (12:15 p.m. ET), prompting closures and evacuations while law enforcers searched the facilities. The FBI was also involved, Knoxville, Tenn., NBC-affiliate WBIR reported. 

By 2:00 p.m. CT, TEMA reported that buildings had been searched in 18 counties, and no explosives had been found.

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The source of the calls is still unknown.

A similar rash of bomb threats against courthouses and other government buildings has affected Oregon, Washington and Nebraska since the beginning of November. None of these threats resulted in discovery of explosives. 

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