Iraqi police from the emergency response division, known as SWAT, inside one of the houses in Fallujah turned into a prison by the Islamic State. The downstairs rooms were used as group cells and the upstairs ones for solitary confinement. (Hawre Khalid/For The Washington Post)

From the outside, there’s not a lot that stands out about the three neighboring houses on this residential street in the Iraqi city of Fallujah.

One is grander than most, with two tall columns straddling its entrance. The others are unassuming and beige, like much of this city, which had been under the control of the Islamic State for the past ­2 1/2  years.

But behind their front doors is a makeshift prison used by the militants to mete out their archaic punishments. It provides a harrowing window into the brutal rule of law that governed here before the city was retaken, a glimpse of its regime of executions, floggings and torture.

Home to many of the Islamic State’s leaders, Fallujah was the first city to fall into the hands of the organization and was a hub for its operations in Iraq. The prison is just one of the remnants of their self-proclaimed caliphate that were left behind by the militants as they died or fled the city and that are now slowly being discovered, allowing Iraqi forces firsthand insight into the group’s inner workings.

As they pick through buildings after steadily recapturing the city over the past month, they are gradually unearthing bombmaking factories, documents, weapons caches and jails like this one — many hidden in regular houses to avoid detection in airstrikes.

Col. Haitham Ghazi, an intelligence officer for the Iraqi police’s emergency response division, also known as SWAT, indicates a room behind a barred door in one of the smaller buildings.

“You can feel the breath of the prisoners inside,” he said.

SWAT officers in a building used by Islamic State militants as a makeshift prison during the ­2 1/2  years they controlled Fallujah. Some inmates appeared to have been summarily executed as Iraqi forces advanced into the city. (Hawre Khalid/For The Washington Post)

The room, perhaps once a living room, is stifling, still thick with the rancid smell of the sweat of those who were incarcerated here.

It’s dazzling daylight outside, but the windows are covered with sheets of metal. The little light that seeps through casts a glow over dozens of little bundles on the carpets — sheets, curtains and scraps of clothes bound together to make pillows. There are dozens, giving an indication of the number of prisoners who were once locked up here.

The hallway outside has been torched. Iraqi security forces say it was like that when they arrived, though pro-government forces appear to be setting some buildings on fire in Fallujah, a claim they deny.

Up a spiraling wrought-iron staircase, the rooms above still contain clothing and other possessions of the family that once lived here, belongings tossed across the floors and beds.

Papers found in the house show that many people were detained after disputes that Islamic State courts had arbitrated, said Ghazi, whose forces discovered the prison. Some were held for stealing, others for minor offenses such as smoking or violating the group’s strict dress code.

Maj. Gen. Thamer Ismail, SWAT’s top commander for the area, said Iraqi forces have found a “a treasure of information” on the group in Fallujah. From here, the Islamic State ordered car-bombing missions in Baghdad and operations as far away as Syria, he said.

His forces found another makeshift prison in Fallujah’s Nazzal neighborhood, he said, but it is smaller than this one in the recently retaken Muallimin district.

“I’m sure there are many more,” he said.

A hole in a garden wall outside that leads to the largest building allowed the jailers to move from house to house without venturing into the street, where they could have been exposed by observation from the air.

A steel sheet has been welded over the marbled entrance to the main greeting room, the first sign that this is no ordinary home. A prison door with steel bars allows access to it and another room that have been joined by a hole in a wall to make a large detention hall. Grills are drilled onto the windows. Blankets and curtains lie scattered on the floor, along with a few dates, which Iraqi security forces believe those held captive were fed. The halls here are airier, perhaps for prisoners accused of lesser infractions.

“There is worse to come,” Ghazi said. “They have no humanity.”

Papers found in the house show that many people were detained after disputes that Islamic State courts had arbitrated, said Col. Haitham Ghazi, whose forces discovered the prison. Some were held for stealing, others for minor offenses such as smoking or violating the group’s strict dress code. (Hawre Khalid/For The Washington Post)

It’s the third house that appears to have been kept for the worst punishments: solitary confinement and torture. A thick, black metal chain with a hook on the end hangs in the stairwell. It has a winch attached.

“They’d hang them here from their legs and beat them,” Ghazi said. “And from this one,” he said, pointing to another hanging hook on a metal cable.

In the upstairs bedrooms, solitary confinement cells had been erected. In one room, there are five cells — a few feet deep and about a foot-and-a-half wide. Their doors and walls are solid metal, with only holes in the top for ventilation.

“How could they even breathe in here in this heat?” Ghazi said. The room next door has five more cells; these are a little larger.

The prison was empty when his forces arrived, Ghazi said. He does not know what happened to most of those who were incarcerated, except for some who appear to have been summarily executed as Iraqi forces advanced.

In the school across the street, their corpses are not hard to find because of the smell of the rotting flesh. In a small courtyard, they lie in a tangled heap in a hole in the ground. There are at least seven bodies here, Ghazi said, adding that there may be other execution sites around the prison.

The bodies appear emaciated, and the skin is blackening as they lie decaying in the heat. Red blindfolds cover the eyes.

The body of one man in sweat pants and a ripped T-shirt is on top. His nostrils are flared, and a pair of scissor handles stick out from his open mouth, the blades plunged into his throat in a final act of barbarity by the jailers before they abandoned the building.

Mustafa Salim contributed to this report.

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