Kalamazoo police say a 14-year-old city resident posted the original online threat of violence that closed Kalamazoo’s Loy Norrix High School on Monday.
Copycat threats prompted district officials to close Kalamazoo Central High School and two middle schools on the same day.
Investigators determined none of the threats were credible, but the investigation is ongoing and more suspects could be identified.
In a statement posted on Kalamazoo Public Schools' website, police asked parents to speak to their students about the seriousness of what happened.
"Joking about a threat or copying a threat that was posted elsewhere is just as serious as a credible threat," the statement said. "Students should remember that if they 'see something, say something.'"
Michigan state law says posting a terrorist threat is a felony punishable by up to 20 years in prison, a fine of no more than $20,000, or both.
Another round of online threats were posted Monday night. The threats targeted Loy Norrix and Kalamazoo Central high schools and Linden Grove Middle School.
However, classes resumed Tuesday at all schools after bomb-detecting dogs swept the buildings. School staff used metal-detecting wands on students as they arrived, and there was an increased police presence at schools across the district.