A Syrian rebel carries an improvised explosive device in 2013. (Karam al-Masrikaram al-Masri/AFP/Getty Images)

Islamic State fighters fleeing their self-declared capital in Raqqa, Syria, have left behind an unprecedented array of deadly improvised explosive devices, U.S. officials say, forecasting what is expected to be a years-long effort to purge them.

Only about 100 militants remain in the city, but the sophisticated explosives they have rigged are slowing efforts to declare a full victory there and could pose grave challenges for Raqqa’s residents as they begin to rebuild after the months-long battle to expel the Islamic State.

“The fight in Raqqa is as much about the fight against IEDs as a fight against ISIS,” Army Col. Ryan Dillon, a spokesman for the U.S.-led coalition in Syria and Iraq, told Pentagon reporters by teleconference Tuesday. He echoed remarks made recently by a commander with the Syrian Democratic Forces, the principal group opposing the Islamic State there.

The weapons range from hastily rigged bombs to hidden pressure plates and motion-activated explosives. They have been found under floors, concealed in ovens and drawers, and attached to corpses and children’s toys.

In at least one instance, a teddy bear was rigged to explode.

The threat lurks with every step made by troops fighting to liberate the city and by the Raqqa Internal Security Force, a local policing unit of about 1,600 that will try to keep the peace there once the offensive ends. Its commander, Idris Khan, was killed by an IED strike Monday. Two SDF soldiers also died in the blast, Dillon said.

“The use of IEDs and explosives is more dense than other cities we’ve seen in this campaign,” including the Iraqi cities of Mosul, Ramadi and Fallujah, Dillon told The Washington Post in an interview. All those cities were heavily mined by Islamic State fighters looking to kill coalition troops sweeping through them to reassert government control.

Surpassing the scope of rigged munitions in Mosul would be significant. Following its liberation, United Nations and U.S. State Department assessments concluded that it could take more than a decade to remove the remaining explosives. It is unclear when such assessments will be made for Raqqa.


A Syrian Democratic Forces fighter stands next to damaged buildings in Raqqa on Sept. 25. (Rodi Said/Reuters)

The U.N. estimated that 300,000 people have fled Raqqa since before the offensive there began in April. Between 10,000 and 25,000 remained as of August; exact figures are impossible to calculate.

Soon, the State Department will lead five multiagency teams into Raqqa, where they will help identify and clear munitions and provide humanitarian assistance, an official there said on the condition of anonymity to discuss the matter openly. As part of that, officials will educate returning residents about the weapons’ hazards, the official said.

They face a problem three years in the making. Bomb builders have circulated lessons and best practices from Iraq and Afghanistan during the reign of ISIS in Raqqa, creating a de facto laboratory for experts to hone their craft.

That expertise will also challenge members of Raqqa’s police force, who receive 10 to 15 days of general training that includes basic counter-IED instruction, Dillon said. It does not include how to defuse and remove the weapons. Some receive additional training focused on such skills, he added, but he was unsure whether the Pentagon is providing any specialized safety equipment such as bomb suits or robots.

The Post's Louisa Loveluck explains the significance of U.S.-backed forces claiming victory over the Islamic State in Raqqa, Syria. (Sarah Parnass,Joyce Lee,Osman Malik,Louisa Loveluck/The Washington Post)

The lack of experience among the forces concerns Matthew Hefti, a former Air Force bomb technician with two deployments each to Afghanistan and Iraq. Bombers often target what troops call fatal funnels — confined places such as hallways, stairwells and doorways. So those tasked with clearing bombs will need to study those places to identify where the militants may have safely ventured.

“You have to be unpredictable,” Hefti said. “You have to do the opposite of what the bomber thinks you’ll do.”

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