Authorities are investigating a string of bomb threats affecting four historically Black universities in the Washington region.

Morgan State University, Howard University, Coppin State University, and the University of the District of Columbia are among multiple historically Black institutions across the U.S. targeted by bomb threats early this week. On Tuesday, the first day of Black history month, all four universities reported receiving bomb threats early in the morning that forced most of them to close temporarily.

It’s the second time since the start of the year that HBCUs have received bomb threats. Bowie State University, in Maryland, also reported a bomb threat on Monday, and has since reopened.

As of Tuesday morning, access to Morgan State’s campus is closed, and all instruction has shifted online, a spokesperson tells WAMU/DCist. UDC’s campus is open, a representative says. At Howard, D.C. police issued an “all clear” following the threat, and a shelter-in-place directive was lifted as of 7:30 a.m., the university tweeted. Howard was targeted by another threat Monday, as well as in early January.

Morgan State University President David Wilson wrote in a letter Tuesday morning that a search for explosives is ongoing, but nothing has been found on campus thus far.

It’s not yet clear whether the bomb threats are connected. In a statement, the FBI said it is “aware” of the threats and is “working with our law enforcement partners to address any potential threats.” White House Press Secretary Jen Psaki addressed the threats Monday, calling them “disturbing.”

In his Tuesday morning letter, Morgan State University President Wilson urged students, staff, and faculty to “stay strong” and “remain resilient.”

“Morgan is one of the most historical and consequential universities in the nation. Our history has been one where we have endured all kinds of challenges and disruptions, but we have always emerged stronger,” the university president wrote.