Some U.S. Historically Black Colleges and Universities (HBCUs) went into lockdown on Thursday after they received reports of threats against them that were probed by authorities, the educational institutions said.

The FBI said it took those concerns seriously but added that it had no indication of a credible threat, but was aware of "hoax threat calls" to a number of HBCUs.

Alabama State University, Hampton University, Virginia State University, Southern University and A&M College, and Bethune-Cookman University locked down their campuses and cancelled classes on Thursday, the institutions said in separate statements.

The statements from the universities did not provide details on the nature of the reported threats. As the day advanced, the universities lifted some restrictions and said no injuries were reported.

HBCUs have also previously faced threats, including simultaneous bomb threats against a dozen such institutions in February 2022. Over the years, U.S. intelligence agencies have warned of threats posed by domestic criminal and extremist groups.

"The violent threats made today against several Historically Black Colleges and Universities throughout the nation are despicable and yet another indication that the explosion of hateful extremism is out of control," Democratic U.S. House of Representatives Minority Leader Hakeem Jeffries said.

Rights advocates condemned the threats.

Hampton University in Virginia said it received notice of a threat, which led to the cancellation of campus activities for Thursday and Friday.

"Enhanced security measures are now in effect across campus," it added.

Alabama State University had suspended campus activities for Thursday after a reported threat. It said later in the afternoon, it received an "all clear" from law enforcement.

"While the immediate threat has been resolved, all non-essential day-to-day operations remain suspended for the remainder of the day, and the campus is still closed to the public," it said, adding it was asking students to shelter in place in their residence halls.

Southern University and A&M College in Louisiana lifted a lockdown that it said was caused by a campus safety threat.

It added that campus activities and classes will remain cancelled on Thursday, Friday and through the weekend.

Virginia State University, which went into a lockdown, also lifted some restrictions and said increased security will remain on campus.