In Burwell, a town north of Kearney, officials held an assembly for students and parents after authorities say a "hit list" was delivered to city hall.
Officials say the "hit list" was delivered last Thursday. It contained more than a dozen names of school administrators, teachers and students.
School officials called off school Tuesday to prepare new security measures.
According to the Burwell Public School District website, they met Wednesday morning at Burwell Elementary and Burwell Junior–Senior High School to talk about heightened security measures. Classes have resumed.
Among the security changes being imposed, the school junior-senior high school will become a closed campus. All bags being taken into the school will be checked, and all high school students and staffers must park in the high school parking lot. An increased police presence also is expected.
No arrests have been reported as officials and investigators try to find out who sent it.