Three weeks after President Donald Trump ordered U.S. forces to pull out of northern Syria, American spy agencies are seeing disturbing intelligence. Turkish-backed militias, armed by Ankara, have killed civilians in areas abandoned by the U.S., four U.S. military and intelligence officials tell TIME. The officials say they fear that the militias committing those potential war crimes may be using weapons that the U.S. sold to Turkey.

These officials say they are concerned that worse could lie ahead. Turkey and its allies are deploying larger forces and bringing more significant weapons to the field than would be necessary to complete their publicly-stated mission. They have said that they plan only to maintain a security zone along a 18-mile wide ribbon of land south of Turkey’s border with Syria. “They’re far more than the Turks need to conduct the operations they’re supposed to be conducting,” one of the U.S. officials told TIME.

Fighting between Turkish and Syrian forces continued on Monday in the security zone as members of the Kurdish forces built defensive positions along the zone’s border inside Syria, one U.S. official told TIME. Two officials confirmed that those trenches and observation posts appear to be preparations for any Turkish-led advance into Kurdish-held territory after the ceasefire expires on Tuesday.

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