President Trump announced Saturday morning he was heading to Dover Air Force Base in Delaware, where the remains of four Americans killed in an attack in Syria on Wednesday will be returned.

"Will be leaving for Dover to be with the families of 4 very special people who lost their lives in service to our Country!" Mr. Trump tweeted Saturday morning. The visit was not listed on his official schedule.

U.S. Central Command said in a statement Wednesday that an apparent explosion killed two service members, a Department of Defense civilian, and a Pentagon contractor while they were "conducting a local engagement in Manbij, Syria." Three service members were also injured.

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"Initial reports indicate an explosion caused the casualties, and the incident is under investigation," the statement said.

Those killed were identified as Jonathan Farmer, Shannon Kent, Scott Wirtz and Ghadir Taher.

The Islamic State of Iraq and Syria (ISIS) claimed responsibility for the attack, which came just weeks after Mr. Trump declared ISIS defeated and said U.S. troops were coming home. The decision to abruptly withdraw from Syria resulted in the resignation of two administration officials, Defense Secretary Jim Mattis and Syria Special Envoy Brett McGurk, who said in December that the end of ISIS will be a long-term initiative, and "nobody is declaring mission accomplished."

Mr. Trump told reporters Saturday morning than when he took over, "Syria was loaded with ISIS," and the situation had improved during his time in office.

"At some point you want to bring our people back home," he said.

The U.S. has about 2,000 troops still in Syria, and Mr. Trump has ordered a withdrawal that is expected to be completed within four months.

Overnight Saturday, American and coalition aircraft continued pounding ISIS positions in support of U.S. troops and their allies on the ground in Syria, CBS News' Charlie D'Agata reported. At the same time, the terror group issued a warning that there will be more suicide bombings targeting American forces.

While ISIS may be fighting for survival on the battlefield, it still has the ability to launch counterattacks.

The bombing in Manbij appears to confirm what many fear: that ISIS is already transitioning from territorial force to underground terrorist network. And it's a tragic reminder of the lethal threat that ISIS still poses in Syria as the Trump administration begins troop withdrawal, D'Agata reported.