CHICAGO (CBS) — A day after security was stepped up at Glenbrook North High School when a student found a message threatening a shooting, students found a second frightening message carved into the seat of a desk.

Glenbrook North principal Dr. John Finan confirmed students found the message “Stay out of school 10 4” and an image of a handgun carved into a seat around 9:30 a.m. Thursday.

The school already had increased security in place, after another threatening message stating “Shooting 10-03” was found carved into a desk Wednesday afternoon.

RELATED: Northbrook School District Investigating Possible Threat

The school and Northbrook Police are investigating both messages.

Starting Wednesday evening, the school began checking all students’ and visitors’ belongings when they entered the building. Northbrook Police also stepped up patrols on and around campus.

“Our focus remains on maintaining a safe and secure school environment for all students and staff. We appreciate your patience and partnership in these efforts,” Finan said in an email to parents on Thursday.

Finan said all school events on Thursday will continue, with added security in place, and officials will continue screening students and visitors as they enter the building.

Anyone with information on the incidents should contact the school at 847-509-2462, or by sending a test to 844-823-5323, with “GBN Help” and a message.