(Reuters) - A U.S. Navy servicewoman and a former Navy SEAL are among the four Americans killed in a suicide bombing this week in northern Syria that the United States believes was likely carried out by Islamic State, U.S. officials said on Friday.

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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.

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.

Kent joined the Navy in 2003 and in her years of service received nearly one dozen honors, including the National Defense Service Medal and two Joint Service Commendation Medals, Navy officials said in a statement.

Wirtz, a Navy SEAL for 10 years, had been employed by the Defense Intelligence Agency since 2017 and completed three deployments for the agency in the Middle East, the agency said.

Army officials did not immediately respond to requests for more details on Farmer.

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 Dec. 19 decision to withdraw all 2,000 U.S. troops from Syria, declaring Islamic State had been defeated there.

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.