A threatening note was found Thursday evening on a bathroom wall at Edmond Memorial High School, the principal told parents in an email.
The message was quickly removed and a report was made to Edmond police, school officials said.
The investigation began Thursday night because the message was discovered after school hours.
Edmond police will provide an increased police presence on campus and will assist in the investigation.
Anyone with information is asked to call Edmond police at 405-359-4338.