WATERLOO, Iowa (AP) — Waterloo school district officials say a threat on social media has forced them to cancel school Wednesday at the district’s three high schools. The officials said in messages sent to parents that they learned Tuesday evening of the threat that referenced West and Expo high schools. They say they worked through the night with police to investigate the posting. No arrests have been reported.

Officials say classes also were canceled at East High, even though it wasn’t mentioned. The three schools’ staff members were told not to go to work. Classes were not canceled at the district’s elementary and middle schools. The officials also say they intend to resume classes Thursday at all three high schools.