Updated 05/06/16 – 11:30 a.m.

CHICAGO (CBS) — A southwest suburban high school was locked down for a couple hours Friday morning, after a parent reported seeing a man with a gun outside.

The Will County Sheriff’s Office said a parent dropping off a student at the start of the school day at Lincoln-Way Central High School in New Lenox saw a man with something in his hand that resembled his gun.

The school was locked down as a precaution, and about 100 students were being kept out while police investigate.

The school reviewed surveillance video from the building, and multiple law enforcement agencies assisted with the investigation, but nothing was found.

Will County Sheriff’s Office spokeswoman Kathy Hoffmeyer said, even if it’s a false alarm, students’ safety is always a concern.

“Whether we believe that it was a cell phone, or whatever we think it may be, it has to be investigated thoroughly to keep the kids and the staff safe,” she said.

Hoffmeyer said police do not want to panic parents, but said any threat of a gun at a school cannot be taken lightly.

“School administrators and staff, along with the Will County Sheriff’s Office, have to take every concern and notification of any concern seriously,” she said.

The lockdown was lifted at 10:30 a.m., and the school resumed a normal class schedule.