US law enforcement on alert in wake of ISIS leader al-Baghdadi's death

The death of the leader of one of the world’s most notorious terror groups may end a particularly brutal saga in the Middle East, but law enforcement and intelligence officials are on alert for possible retaliation from sympathizers or sleeper operatives in the U.S.

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 its “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.

On Friday, before al-Baghdadi’s death was announced, New York and federal authorities -- including the New York City Police Department, DHS and the FBI -- issued a security assessment of the upcoming New York City Marathon that highlighted the threat of ISIS-related attacks, among other categories.

In it, authorities detailed how ISIS sympathizers disseminated propaganda urging attacks in New York and cited as examples recent foiled terror plots in the U.S., including a plot in New York to attack bystanders with a knife and one near Washington, D.C., that involved using a van to run over pedestrians.

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ISIS has typically encouraged followers and sympathizers abroad to carry out attacks in areas they live in using whatever means available to them.

In 2015, 14 people were killed and 22 were injured in San Bernardino, California, after a local married couple opened fire and attempted to bomb a workplace Christmas party. Authorities later said the couple were not directly affiliated with ISIS, but instead had been radicalized over years of consuming the terror group’s propaganda online.

And in 2017, a man ultimately killed eight people in New York after driving a rented truck about three-quarters of a mile on a bicycle path, aiming for cyclists and pedestrians. Authorities told ABC News at the time that the suspect left a note proclaiming his attack was on ISIS’s behalf.

ABC News' Aaron Katersky and Josh Margolin contributed to this report.