A posted threat on the Internet prompted a community college in Kalamazoo, Mich., to close until next week. All public schools in the 12,000-student district in Yuba City, Calif., and neighboring Marysville locked their doors while authorities searched for a man they said threatened a massacre.

And Schaumburg High School in Chicago's northwest suburbs was ordered evacuated Thursday afternoon, after rumors surfaced that someone planned to plant a bomb there Friday. Three students were led away in handcuffs after authorities found them in possession of gloves, wires and drill bits, materials that could be used in making a bomb.

The evacuation took students by surprise because it came about two hours after officials announced over the school's public address system that police and dogs would search the building after school for "suspicious materials" as a precaution.

Thursday's activity marked the third and busiest day of copycat crimes nationwide since 32 students and faculty members were shot and killed at Virginia Tech.

"The problem is events like these attract a lot of attention, and rightly so. Excessive attention to the Virginia Tech shootings drives the copycats, and that is exactly what is going on now," said Jack Levin, a professor of sociology and criminology at Northeastern University in Boston.

"I think the copycat thing in general has a short lifespan, maybe a few weeks. Then it will be over for most students," Levin said.

For now, though, the reverberations from Virginia Tech are making school officials across the nation edgy and keeping local police agencies on alert for what, in most cases, prove to be hoaxes.

In Michigan's Ann Arbor Township, sheriff's officials responded to a threatening message that had been written on a bathroom wall at Gabriel Richard High School.

"It was pretty clear early in the process that it was a hoax ... and we had information as to who wrote it," said Cmdr. David Egeler of the Washtenaw County Sheriff's Department.

But before deputies arrested a 17-year-old male student, word of the threat spread to parents and school officials canceled classes for the day. In the meantime, news of possible trouble at the high school spread through the community and other area schools locked their doors and kept students inside. Egeler said the situation "turned into a media storm."

"I remember Columbine, and no, I'm not surprised by this," Egeler added.

Officials at Kalamazoo Valley Community College said they were contacted by Michigan State Police about a "very specific threat" to the college, and they closed the school through the weekend. Michael Collins, the school's vice president, said in a statement that they are "actively working with local law enforcement agencies to resolve the situation."

At Pittsburgh's Point Park University, officials imposed a partial lockdown of less than an hour after a man was shot near the campus Thursday morning. In the Pittsburgh area, the Allegheny Valley School District canceled classes for 1,200 students after administrators were notified of a safety threat. School was scheduled to resume on Friday.

Some incidents have resulted in arrests. A 14-year-old high school student in St. Augustine, Fla., was charged with a felony Thursday for sending an e-mail in which he allegedly threatened to kill 100 people. The St. Johns County Sheriff's Department said the youth was being held in a juvenile detention center.