(AP) — A menacing letter sent to Burwell’s city hall contains more than a dozen names of Burwell school administrators, teachers and students.

The letter didn’t say would happen to any of those listed, but Garfield County Attorney Dale Crandall says law enforcement and school officials “are taking the threat seriously.”

Investigators have been trying to find out who sent the letter, which was delivered Thursday. Classes at Burwell Junior-Senior High School were canceled Tuesday, and an increased police presence is expected there until the case is solved.