Sunni militants captured the Mosul dam, the largest in Iraq, on Thursday as their advances in the country’s north created an onslaught of refugees and set off fearful rumors in Erbil, the Kurdish regional capital.

Residents near the dam and officials in the region confirmed that the Islamic State in Iraq and Syria, or ISIS, held the dam, a potentially catastrophic development for Iraq’s civilian population.

The dam, which sits on the Tigris River and is about 30 miles northwest of the city of Mosul, provides electricity to Mosul and controls the water supply for a large amount of territory. A report published in 2007 by the United States government, which had been involved with work on the dam, warned that should it fail, a 65-foot wave of water could be unleashed across areas of northern Iraq.

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