Police in Detroit and neighboring communities are warning young people against threatening violence at schools.
About two dozen area schools were closed Thursday.
One Detroit school was placed on lockdown after a threat was found on social media and two suspicious packages were discovered on school property.
Police later determined the packages posed no threat.
"I view this as social terrorism, because it creates social unrests. It instills fear," Detroit police Chief James Craig told reporters Thursday. "We're going to pull the full force of law enforcement to focus on you. So, the threats stop and they stop now."
Schools across the United States have experienced a rash of threats following the shooting deaths last month of 17 people at a Florida high school by a former student.
There have been roughly two dozen school threats in Detroit over the past 24 hours, said Craig, adding that four males — ages 14 to 17 — have been arrested in separate cases in recent days.
More arrests are expected, he said. Charges can include threat of terrorism and using telecommunications to make a false report.
"We're very sophisticated in how we conduct our investigations," Craig said. "Some young people believe they have apps to make them anonymous. We know how to overcome that."
Parents also were warned.
"This is not a joke," said state police Lt. Mike Shaw. "If mom or dad knew about it, we're coming for you, too."
Earlier Thursday, state law enforcement and education leaders called for Michigan's Legislature to spend $100 million a year to staff schools with more police and counselors in light of the massacre at Marjory Stoneman Douglas High School in Parkland, Florida.
They also asked for $20 million in state funding annually to harden schools' security infrastructure and proposed making it mandatory that the security of every school building in the state be assessed at least once a year during walkthroughs by officers.