WASHINGTON (Reuters) - Two U.S. troops, one Defense Department civilian and a contractor supporting the U.S. military in Syria were killed on Wednesday in an apparent suicide bomb attack in the city of Manbij, the U.S. military’s Central Command said.

An additional three U.S. troops were injured, it said.

“Initial reports indicate an explosion caused the casualties, and the incident is under investigation,” Central Command said in a statement.