A wave of clown threats and sightings swept across the Chicago area and northwest Indiana on Monday and Tuesday.

The first known threat was posted on Instagram at 8 p.m. Monday against Lincoln-Way Central High School in Will County, according to a statement from Will County sheriff’s office. The threat was later discovered to be a hoax by a 14-year-old male student enrolled at the school.

A clown was also sighted Monday night near 111th and Harlem Avenue in Worth, according to a Facebook post made by the Chicago Ridge Neighborhood Watch group. The clown was also seen walking on 79th Street in Burbank.

At 10:43 p.m., someone posted on Twitter a threat against all Chicago Heights schools, according to Chicago Heights police. An investigation has been launched into who created the Twitter handle.

About 8:45 a.m. Tuesday, a group of parents were walking their kids to school in the 6000 block of South Damen in West Englewood when someone dressed as a clown ran towards them, according to Chicago Police. The group ran into a nearby school and reported the incident.

At 9:25 a.m., an electronic threat was made at a school in the 900 block of South Homan in North Lawndale, police said. CPS is currently investigating the threat.

Another threat made on social media against a Portage Park school Tuesday was later determined to be fake. “There are no credible threats to any school right now,” the 25th District Chicago CAPS Tweeted Tuesday morning. “The Killer clowns are internet fantasy, nothing more.”

Another clown sighting was reported later on Tuesday at the University of Illinois at Chicago, according to the UIC’s Twitter feed. The “malevolent clown was a student doing a performance-art project. No arrest, says UIC police.”

Reported malevolent clown was a student doing a performance-art project. No arrest, say UIC Police. "We take these calls seriously." — UIC News (@UICnews) October 4, 2016

About 1:50 p.m Tuesday in Waukegan, three people dressed in clown masks stood at the edge of Greenwood Elementary School’s property taunting students, according to a statement from Waukegan police.

Students reported that one of the clowns had a knife, one had a briefcase and one had a gun “in his pocket”, police said. It was unknown Tuesday night if the knife was real or if the one clown was armed with a handgun.

The children who saw the clowns also reported they had threatened to return Friday to, “kill them”, police said. It’s possible the three clowns fled in a black SUV. The gender and race of the suspects was also unknown Tuesday night.

3 people dressed as clowns threatened children at Greenwood school today. School was on a brief lockdown. No suspect information available. — Waukegan Police Dept (@Waukegan_Police) October 4, 2016

Greenwood was briefly placed on lockdown until officers were able to check the nearby area, police said, who are currently providing extra watches during patrol shifts at all local schools. Anyone with information is asked to call the CID Tip-Line at (847) 360-9001.

Threats by people representing themselves as clowns have also been made against Hammond High, Eggers, Gavit, Scott and Morton in Hammond, according to a statement from Hammond Police Lt. Richard Hoyda.

“So far there have been no reports of confirmed clown sightings in the City of Hammond and at this time the information appears to be solely based on Facebook,” Hoyda said.

Anyone with information about the threats is asked to call police at (219) 852-6326.

According to the Boston Globe, residents in at least 26 states have reported “unsettling encounters” with people dressed as clowns. The reasons behind the clown sightings and threats, which started in South Carolina in August, are not clear.