(CNN) The initial US assessment of the deadly bombing in Syria that killed four Americans is that ISIS was behind the attack, two US officials said Thursday.

One official said it is believed that Wednesday's attack in Manbij was carried out by an ISIS sleeper cell.

ISIS claimed responsibility for the bombing on Wednesday. The ISIS-affiliated Amaq agency said the attack was carried out by a suicide bomber with an explosive vest.

The American deaths included two US service members, a defense contractor and a Department of Defense civilian, the US Central Command said in a statement. Three other US service members were injured in the attack.

Prior to Wednesday's attack, only two US service members had been killed in action in Syria since the start of the campaign in 2014.

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