(Reuters) – The United States on Friday identified three Americans killed in a suicide attack in northern Syria this week that the U.S. government said was likely carried out by the Islamic State militant group.

Army Chief Warrant Officer Jonathan Farmer, 37, of Boynton Beach, Florida; Navy Chief Cryptologic Technician Shannon Kent, 35, identified as being from upstate New York, and Scott Wirtz, a civilian Department of Defense employee from St. Louis, died during the Wednesday attack in Manbij, Syria, the Department of Defense said in a statement.

U.S. Army Chief Warrant Officer Jonathan Farmer, 37, of Boynton Beach, Florida is pictured in an undated photo released by the U.S. Army, in Fort Bragg, North Carolina, U.S., January 18, 2019. U.S. Army/Handout via REUTERS

The Pentagon did not identify the fourth person killed, a contractor working for a private company.

The Manbij attack on U.S. forces in Syria appeared to be the deadliest since they deployed on the ground there in 2015. It took place in a town controlled by a militia allied with U.S.-backed Kurdish forces.

Two U.S. government sources told Reuters on Thursday that the United States views the Islamic State militant group as likely responsible for the attack. Islamic State has claimed responsibility.

The attack occurred nearly a month after President Donald Trump confounded his own national security team with a surprise decision on Dec. 19 to withdraw all 2,000 U.S. troops from Syria, declaring Islamic State had been defeated there.

U.S. Navy Cryptologic Technician Shannon M. Kent, 35, is pictured in an undated photo released by the U.S Navy, in Washington, DC, U.S., January 18, 2019. U.S. Navy/Handout via REUTERS

If Islamic State carried out the attack, that would undercut assertions, including by U.S. Vice President Mike Pence several hours after the blast on Wednesday, that the militant group has been defeated.

Experts do not believe Islamic State has been beaten despite its having lost almost all of the territory it held in 2014 and 2015 after seizing parts of Syria and Iraq and declaring a “caliphate.”

While the group’s footprint has shrunk, experts say it is far from a spent force and can still conduct guerilla-style attacks. An Islamic State statement on Wednesday said a Syrian suicide bomber had detonated his explosive vest in Manbij.

(Reporting by Gina Cherelus in New York; Editing by Scott Malone and Steve Orlofsky)