Tennessee became the fourth state to deal with similar bomb hoaxes. One targeted 28 courthouses in Oregon and similar threats were reported in Nebraska and Washington this month.
Nine threats were reported in West Tennessee counties - including the Memphis federal building - seven in Middle Tennessee and 14 in East Tennessee, said Tennessee Department of Safety and Homeland Security spokeswoman Dalya Qualls.
No arrests have been made in Tennessee and authorities had searched about 14 courthouses by Tuesday afternoon.
Jeremy Heidt, a spokesman for the Tennessee Emergency Management Agency, said the threats were made by telephone to court clerk offices. "It's been going on all morning," said Heidt.
The threats to courthouses in Ashland City and Clinton were made by men.
"It was just a man's voice saying simply if this is the Cheatham County Courthouse you're about to be blown up, and it startled me. So I just said I'm sorry. I tried to get him to repeat himself and that's when it just cut off and I thought `Oh my,'" Cheatham County deputy clerk Rebecca Nicholson told WTVF-TV.
Anderson County Clerk Jeff Cole told The Knoxville News Sentinel that a man's voice simply said "There's a bomb in the building" and hung up.
The FBI is working closely with state authorities and will pursue federal charges if necessary, said Joel Siscovic, the bureau's spokesman in Memphis.
The federal building in Memphis houses the federal court and offices for the Department of Justice, U.S. Army Corps of Engineers, and U.S. Rep. Steve Cohen. It was cleared around 9 a.m. for a couple of hours while authorities checked it.
Police temporarily blocked a trolley line that ran by the federal building in Memphis and brought in a dog to search the building before letting people return around 11 a.m.
According to Memphis police spokeswoman Karen Rudolph, a woman called police around 8:30 a.m. and said she had information that someone was going to blow up the county criminal justice center, the federal building and a main U.S. Post Office in the city.
All three locations were searched and nothing was found, she said.
(Copyright 2012 by The Associated Press. All Rights Reserved.)