Students and staff began returning to Kilo Middle School around 9 a.m. after a bomb threat shut down the school early this morning.

The King County Sheriff’s Office swept the school and its grounds with a bomb sniffing dog and found no threatening devices.

The threat was received via e-mail before 7 a.m. Thursday by an administrator, and officials said it was sent sometime overnight. After discovering the e-mail, Kilo Principal Pat Larson contacted school district security officials, who contacted King County sheriffs. Parents were notified of the threat by the district’s voice messaging system.

Students on their way to school on district buses were rerouted to Thomas Jefferson High School and walkers were sent to Lake Dolloff Elementary school.

Kilo Middle School, 4400 S. 308th St., has a population of about 640 students. District spokeswoman Diane Turner said that schools received a variety of threats, including bomb threats.

“I’m not going to say it’s a common occurrence,” she said. “But it does happen. We have to work closely with police to determine whether it’s a viable threat or not.”

Sgt. John Urquhart said his department will investigate the threat. Kilo is located in unincorporated King County. Bomb threats are often made by phone, which can make it harder to find out who did it. A bomb threat against property or a person is a Class B felony in Washington, punishable by a maximum of 10 years in prison and up to a $20,000 fine.

“This is a huge disruption to the school,” Urquhart said. “We’ll hold whoever it is accountable. These are really pretty common, especially to schools, especially when math tests are coming.”