CHICAGO (CBS) — School District 225 in north suburban Northbrook is taking additional security measures after a student found a frightening message in a classroom.

Glenbrook North High School sent a letter to parents saying a student found the inscription “Shooting 10-03” on a desk.

Nearby Glenview School District 30 issued a letter to parents notifying them of the message discovered at the high school. No threats were found at that school.

The Northbrook Police Department will have officers on patrol at the high school over the next several days.

Police want to remind everyone to stay vigilant and report any suspicious activity.

A separate threat was discovered at Glenbrook South High School last week.