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.

This intelligence has led some U.S. analysts to conclude that Turkey and its allies may be preparing to clear civilian populations from the area, which has largely been controlled by the Kurds, Ankara’s long-time enemies in the region. Turkish President Recep Erdogan told the United Nations on Sept. 24 that he planned to establish a safe zone across the border in Syria, and to resettle some of the 3.6 million Syrian refugees currently sheltered in Turkey. On Oct. 26, Erdogan said Turkey planned to “clear terrorists” from the so-called safe zone if Kurdish forces aren’t gone by 3 p.m. Greenwich Mean Time on Oct. 29.

U.S. officials are worried that a full collapse of the ceasefire on Tuesday, combined with a broad Turkish campaign to remove Kurds from the security zone, will result in wide scale war crimes. They are also worried that a humanitarian crisis, combined with renewed fighting, could accelerate the return of ISIS, which thrives on disorder.

U.S. intelligence agencies retain some sources in Syria, thanks to networks developed during America’s five-year deployment there, and the U.S. military is flying regular drone reconnaissance missions over the area. The scope of U.S. intelligence activity in the region has drawn renewed interest in recent days, in the wake of a U.S. raid on Saturday that killed ISIS Leader Abu Bakr al-Baghdadi. The CIA, as well as Iraqi and Kurdish intelligence officers, tracked the ISIS chief by interviewing the wife of an al-Baghdadi aide and one of his couriers, and by recruiting local spies along the Syrian-Iraqi border.

The intelligence on a potential Turkish offensive in Northern Syria has come from a combination of human and technical sources, the officials tell TIME, and some of it has been shared with U.S. agencies by allied countries. The intelligence on the use of U.S. arms in the attacks on civilians remains inconclusive. The officials, who spoke on condition of anonymity because they were not authorized to publicly discuss the fighting in Syria, declined to provide more details.

This intelligence is emerging as the U.S. struggles to manage the fallout from its precipitous retreat from Syria, which was announced Oct. 13, after Erdogan told Trump that Turkey was about to attack territory in northern Syria where U.S. troops were deployed. Trump gave the Pentagon and State Department no warning of his decision to pull the U.S. out of the area, and no time to plan an organized retreat or to negotiate a handover of territory. That has left U.S. military officials and diplomats scrambling to deal with the situation.

Among the principle challenges: managing the unpredictable Commander-in-Chief, two of the officials say. On Oct. 25, Trump reversed part of his Oct. 13 order to withdraw almost all of the 1,000 American forces from Syria. At the NATO headquarters in Brussels, U.S. Defense Secretary Mark Esper announced that Washington is sending a limited number of armored forces into eastern Syria. He described the redeployment as an effort to prevent the country’s oil fields from falling back into ISIS’s hands.

The announcement was perplexing. Protecting Syria’s oil fields in the Deir al-Zour province, where the troops are deploying, makes little strategic sense, two of the officials involved in the redeployment tell TIME. “The oil fields are small, we blasted them after Daesh [ISIS] seized them, and they will take years to rebuild,” said one official. So why leave forces there to protect them? “Talking about oil was the only way we could talk the President into keeping any U.S. military force in the area,” the official says. On Friday, after the plan to protect the oilfields was unveiled, Trump tweeted, “Oil is secured.”

On Oct. 27, Esper added that the new deployment of troops would not just be guarding oil fields; they would also engage in counterterrorism operations. “Despite Baghdadi’s death, the security situation in Syria remains complex,” Esper said at the briefing Monday. He said that the U.S. forces deployed in Syria “will continue to execute counter terrorism operations while staying in close contact with the Syrian Democratic Forces that have fought along side of us.”

But a residual force does not put the U.S. in a position to respond quickly in the region. If the ceasefire in the north collapses, the U.S. has no plan to intervene. U.S. Central Command had hoped to keep the rest of the 1,000 forces withdrawn from Syria just across the border in western Iraq, where it could continue to target ISIS and deter widespread instability. But the U.S.-backed government in Baghdad vetoed that idea and gave the troops a four-week deadline to leave.

U.S. officials are worried that a humanitarian crisis and renewed fighting in the region will invite a resurgence of ISIS, which operates best in chaotic situations. Many captured ISIS fighters remain in Kurdish custody in northern Syria. Trump appeared to dismiss the danger of a renewed terrorist threat Friday, when he tweeted, “ISIS SECURED”. Esper told reporters at NATO that the U.S. mission remains preventing a resurgence of ISIS.

There is no clear intelligence on how many suspected ISIS fighters have already escaped from prisons guarded by the Kurds in northern Syria, nor how many of the refugees held in camps in the area might have been radicalized during their stay in hardship conditions, the officials tell TIME. The minimum estimate of escapees is more than 100 already, said one of the officials, who added, “That’s a lowball, I’m afraid.”

The officials said their most immediate fear now is that Turkey may use its military incursion into the so-called safe zone as cover for a larger attack against the Kurdish population. “Even if the Kurds continue to withdraw peacefully that doesn’t mean the Turks won’t find a reason to act,” the same official said.

U.S. intelligence officials aren’t the only ones seeing evidence of war crimes. The human rights group Amnesty International reported on Friday that Turkish-backed Syrian forces have committed war crimes, including executions of Kurdish civilians. Some of the acts were photographed on mobile phones. Amnesty International said it had collected evidence from 17 witnesses. It cited reports by Kurdish-led health authorities in the region that more than 200 civilians had been killed by Turkish forces and their allies during the recent fighting and Turkey claimed that 18 of its civilians were killed in a cross border mortar attack.

James Jeffrey, the U.S. special envoy for Syria, told a House of Representatives hearing on Wednesday that the administration has asked the Erdogan government to answer questions about possible war crimes committed by Turkish forces and their allies. “We have several incidents we consider war crimes,” Jeffrey said. Both the Kurdish Red Crescent and the U.K.-based Syrian Observatory for Human Rights have reported apparent human rights abuses during the Turkish offensive.

Congress is pushing for more details on the situation in Northern Syria. On Oct. 25, four U.S. senators sent Secretary of State Mike Pompeo a letter seeking information by Nov. 1 on reports that Turkish-backed forces used white phosphorus, a chemical weapon, against the border town of Ras al-Ain, as first reported by Foreign Policy on Oct. 17. In their letter, the four—Maryland Democrat Chris Van Hollen, Vermont Democrat Pat Leahy, Connecticut Democrat Richard Blumenthal, and Tennessee Republican Marsha Blackburn—said the report has “been substantiated by additional outlets, and the Organization for the Prohibition of Chemical Weapons has stated that it is aware of the situation and is collecting information with regard to the possible use of chemical weapons.”

The Senators also asked Pompeo if any weaponry supplied to Turkey by the U.S. has been used “in connection with the Turkish offensive in Syria, including in connection with the use of chemical weapons.”

For now, the best news for those hoping that international forces might quell the violence in Northern Syria has come from an unlikely source: Moscow. The Russian Defense Ministry said on Oct. 25 that Moscow has sent an additional 300 military police to the region and airlifted 20 armored vehicles to equip them. “The bad news is that they came from Chechnya,” said one of the officials. Russia brutally suppressed a series of Islamic uprisings in the predominantly Muslim Chechen Republic after it declared independence from Russia in 1991.

U.S. officials are left hoping those Russian forces are better than nothing. “They’re all that stands between the Turks and the Kurds now that we’re out,” one official says.

Write to W.J. Hennigan at william.hennigan@time.com.