CNN  — 

Once again, a bomb threat has derailed classes at Howard University – one of more than a dozen historically Black colleges and universities targeted with bomb threats during Black History Month.

“A shelter-in-place order has been issued following a bomb threat against the main campus,” the university’s Department of Public Safety tweeted Monday.

“Campus perimeter is being swept by municipal and federal law enforcement officials.”

The Metropolitan Police Department of Washington, DC, responded. As of Monday afternoon, “Howard University has been cleared with no hazardous materials found,” MPD spokesperson Brianna Burch said.

The university has endured at least four bomb threats in the past two months.

“We’ve had these challenges before,” President Wayne Frederick said shortly after a bomb threat on February 1 – the first day of Black History Month.

But “since I’ve been here (as a student) in 1988, it has not been this widespread and also, I think, this overt.”