CHICAGO (CBS) — Two 13-year-old boys were facing charges, and extra officers were in place at a north suburban middle school on Monday, after threatening messages were posted on social media.

Lake County Sheriff’s police said several students at Fremont Middle School in Mundelein received threatening messages on Sunday, referencing the 1999 massacre at Columbine High School in Colorado.

An investigation determined the messages were sent by a 13-year-old Fremont student from Round Lake. After speaking to him and his parents, the teen admitted sending the messages as a “joke,” police said.

Detectives found a classmate, also a 13-year-old boy, had provided the suspect with ammunition, but had not participated in the threatening messages.

Police said investigators also learned the teen who sent the threatening messages had stolen a gun from a relative one year ago, but then threw it into a body of water. The relative who owned the gun later recovered it, but neither of the boys police questioned had access to the gun at the time the threatening messages were sent.

The first 13-year-old was charged with disorderly conduct for posting the threats on social media, unlawful possession of ammunition, and unlawful possession of a firearm, in connection with stealing his relative’s gun last year.

The second teen was charged with unlawful possession of ammunition for providing the other boy with the ammunition.

Both boys were being held at the Hulse Juvenile Detention Facility.

According to the Sheriff’s office, extra police were present at Fremont Middle School on Monday as a precaution.

Fremont School District 79 officials said police confirmed there were no substantiated threats, and no safety concerns at the school. Classes were being held as normal on Monday, and school staffers and counselors were available to answer any questions students might have about the threats.