School officials at Oak Park and River Forest High School closed the campus for about an hour Tuesday morning as law enforcement officials investigated rumors of a threat at the school.
The lockdown was lifted shortly before 11 a.m. after police said there was no threat to the high school.
Just before 10 a.m., movement in the building was limited and the campus was closed as police investigated a potential threat, an alert from Karin Sullivan, director of communications and community relations for the school, read.
No visitors were allowed to enter the campus, Sullivan said.
A student saw a social media posting she believed indicated a potential threat of a weapon on campus, she said.
"As soon as the matter was brought to our attention, we decided that the most prudent course of action was to close the campus until police officials could complete their investigation," Sullivan said.