Authorities are investigating after a threat triggered a lockdown at a school in northeast South Dakota.

The threat was made against the Milbank School District Friday morning, according to a statement from the district.

The school notified law enforcement, including SWAT and the FBI. Law enforcement officers swept the site and found no threats or devices.

"At no time were any threats found inside the building, and student and staff safety remained our top priority throughout the process," the statement said. "We appreciate the cooperation and professionalism of our students and staff during this situation."

Investigators are working to trace the source of the call.