MOSUL, Iraq (Reuters) - The boy’s fear was palpable as Iraqi soldiers brought him blindfolded before an intelligence officer in a house on the northern edge of Mosul.

Iraqi army soldiers detain a person suspected of belonging to Islamic State militants during a battle with Islamic State militants in Arabi neighborhood in Mosul, Iraq, January 26, 2017. REUTERS/Muhammad Hamed

“How long were you with Daesh (Islamic State)?” colonel Amer al-Fatlawi asked the boy in front of him.

“Twenty days, sir,” replied the 17-year old submissively.

The boy appeared harmless, but Fatlawi, the head of intelligence for the 16th division of the Iraqi army, suspected he may pose a latent threat after Islamic State’s days of ruling over vast swathes of territory come to an end.

More than two years after the militants took over Mosul and proclaimed a caliphate for all Muslims, Iraqi forces backed by a U.S.-led coalition have retaken the eastern half of the city, and now have the west in their sights.

Although thousands of militants have been killed since the start of the campaign three months ago, Islamic State is expected to live on, going back underground and reverting to its insurgent tactics of old.

That means the enemy will be less visible to Iraqi forces, and the fight against it more covert.

“They have planted him as a sleeper cell,” Fatlawi said when the boy was out of earshot. “He will be a secret informant for Daesh.”

Slight and wearing jeans, the boy said he was one of a group of some 150 men who gathered at a local mosque around one year ago and were taken to a training camp nearby.

The daily routine involved waking at dawn for prayer, followed by breakfast, physical exercise, lessons in Islamic doctrine and how to use a kalashinkov.

After three weeks, the recruits were allowed to go home on break: “They told us to come back, but I didn’t. I was scared,” said the boy.

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Fatlawi was not convinced: “They all say they quit,” he said, skeptically. “We will interrogate him and get information. If you know your enemy, he is easy to find.”

As Iraqi forces rout Islamic State from the east, they are learning more about the workings of the militant group, which left behind a formidable paper trail.

On Fatlawi’s desk was a stack of documents recovered from Islamic State bases in northern Mosul, including diagrams for making unmanned aircraft and two Russian passports from which the pages containing personal details had been torn out.

The passports appeared unused, except for a single stamp upon entrance to Turkey in 2013.

There were also internal communications sent from senior Islamic State members to mid-ranking commanders, with instructions not to use earphones whilst on duty, and to smile and speak nicely to their subjects in order to “increase affection amongst all”.

“DYING OF FEAR”

Iraqi forces have relied on locals to inform on those who collaborated with Islamic State as they enter each new district, but the security apparatus is beginning to conduct more systematic checks.

In one of the last neighborhoods to have been cleared on the eastern side of the river, billboards still welcome visitors to the Islamic State, and the corpses of militants lying in the road have not been rotting for long enough to smell.

“He is called Abu al-Harith,” said captain Aras, identifying a militant who drew his last breath near a dumpster as Yemeni before turning away to retch.

On a street nearby, children play as though nothing has happened, and men emerge from their houses acclimatizing to the new reality of Iraqi soldiers patrolling the streets instead of Islamic State.

One resident still had a full beard and wore his trousers tucked into his socks, in keeping with the dress code imposed by Islamic State -- modeled on the way the Muslim Prophet Mohammed is thought to have dressed in 632 AD.

The rules were enforced by the Hisba or vice squad, which cut people’s trousers if they fell below the ankle, and whipped or fined those who trimmed their beards.

“Why haven’t you shaved your beard?” asked Captain Aras angrily, ignoring the man’s protest that he was a devout Muslim. “Shave it all off!” Another soldier knelt down, un-tucked the man’s trousers and cut the elastic bottoms with a knife.

A trail of spent bullet casings marks the route by which the militants were driven back by Iraqi forces down a narrow alley towards the river Tigris.

As the army advanced, the militants forced some residents out of their homes, using the dwellings to mount a futile defense of the area, and torching them as they withdrew.

“They were dying of fear,” said Abu Malek, describing how a small group of fighters had threatened him to get out of his house at gunpoint. “They were all young children”.

In the entrance to Rakan Abu Omar’s house, which was also occupied and then torched by Islamic State, the blood of a mortally wounded fighter who was dragged in off the street has yet to dry.

Locals said they saw the militants put his body in a white plastic bag and ferry it across the river by boat.

It was a hopeless end. The young prisoner’s fate is not yet so certain.

“Don’t be afraid,” Fatlawi said, patting him reassuringly on the back. The boy could not see him wink.