With the death of the leader of notorious terror group ISIS in the Middle East, law enforcement and intelligence officials are on alert for possible retaliation from sympathizers or sleeper operatives in the U.S., reports ABC News.

Abu Bakr al-Baghdadi, the founder and leader of the Islamic State of Iraq and Syria—or ISIS—died on Sunday as a result of a raid executed by U.S. special forces in Syria. Authorities say, however, that despite a diminished physical caliphate and vacancy at its highest rank, ISIS has still been able to influence vulnerable populations abroad via online message boards and social media groups.

The Department of Homeland Security said in a statement Monday that it's "operating at a heightened state of vigilance" as it and other agencies assess the potential for retaliation against al-Baghdadi's death from terror cells or ISIS sympathizers in the U.S., though a specific threat has yet to be identified.

"Our security posture will remain agile; we will continue to mitigate and respond to the ever-evolving threat landscape," a DHS spokesperson said.

Authorities' concerns about reaction to al-Baghdadi's death come amid an already elevated level of alert for terrorist attacks.

The FBI recently released information that said that one in five active terrorism investigations in the U.S. involved ISIS followers or sympathizers spanning virtually every state.