The U.S. Naval Academy in Annapolis, Maryland, was placed on a security lockdown on Thursday while military personnel and local law enforcement investigated reports of threats to the installation, the Navy said on the academy's Facebook page.

There was no immediate official word on the nature of the reported threat. But Fox News, citing multiple sources inside the academy, said a former midshipman who had been expelled from the school had returned to campus armed with a weapon.

According to the Fox News report, gunshots were heard inside Bancroft Hall, which houses the midshipmen.

According to the Facebook statement from Naval Support Activity Annapolis, the "base is on lockdown out of an abundance of caution." It said Navy support personnel, in coordination with local law enforcement, was "responding to reports of threats made to the Naval Academy." 

Aerial news footage showed police officers with long guns and body armor running outside a campus building.

In addition, at least five historically Black colleges locked down campuses, canceled classes or ordered students to shelter in place after receiving “credible threats” of violence on Thursday. Many of the precautionary measures are no longer in place.

The FBI said the threats were a "hoax."

"The FBI is aware of hoax threat calls to a number of Historically Black Colleges and Universities," the agency said in a statement. "The FBI takes these threats very seriously because it puts innocent people at risk. While we have no information to indicate a credible threat, we will continue to work with our local, state, and federal law enforcement partners to gather, share, and act upon threat information as it comes to our attention."

Alabama State University, Clark Atlanta College, Hampton University, Southern University and Virginia State University were among those affected.