This is stated in the message of the U.S. Department of Homeland Security.

The United States is facing increased threats from extremist groups at home and abroad, as evidenced by the hostage-taking at a Texas synagogue last month and threats to plant bombs at many colleges and universities where historically the majority of students were African-Americans, the U.S. Department of Homeland Security said.

The warning came after some schools in the United States canceled classes and issued self-isolation orders last week. In the end, the investigators failed to find any explosives.

“Threats directed at colleges and universities, where historically the majority of students were African-Americans, as well as other colleges and universities, Jewish institutions and churches, are of concern and may inspire extremist terrorist groups to violence,” the bulletin of the Department of Homeland Security says.

Last month, an armed man took four people hostage at the Beth Israel Synagogue in Colleyville, Texas, including its rabbi Charlie Cytron-Walker.

The ten-hour standoff ended in a shootout, all four hostages were released, and the suspect was killed.

A few months ago, the U.S. intelligence community warned of the threat from extremists, such as white supremacists, who would try to commit mass murder.

The agency also warned that the Islamic State or its affiliates may make public calls for retaliation because of the raid by U.S. special forces in Syria last week, which killed the group’s leader Abu Ibrahim al-Hashimi.