He would wander the streets of occupied Mosul by day, chatting with shopkeepers and ISIL fighters, visiting friends who worked at the hospital, swapping scraps of information. He grew out his hair and his beard and wore the shortened trousers required by the extremists. He forced himself to witness the beheadings and stonings, so he could hear executioners call out the names of the condemned and their supposed crimes.
By night, anonymously from his darkened room, Mosul Eye told the world what was happening. If caught, he too would be executed.
But after more than three years, his double life has grown too heavy to bear. He misses his name.
His secrets consume him, sap energy he'd rather use for his doctoral dissertation and for helping Mosul rebuild. In conversations with The Associated Press, he agonised over how to end the anonymity that plagues him. Then, he made his decision.
Mosul Eye is Omar Mohammed, a historian, scholar and blogger. He is 31.
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Read more:
[ The liberation of Mosul: Victory at a price ]
[ Battle-weary civilians tread a perilous path out of Mosul ]
[ Families flee past Mosul's destroyed Al Nuri mosque - in pictures ]
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The revelation of Mr Mohammed's identity is for his thousands of readers and followers, for all his volunteers in Mosul who have been inspired by a man they have never seen. But above all, it is for the brother who died in the final battle and for his grieving mother.
"I can't be anonymous anymore. This is to say that I defeated ISIS. You can see me now, and you can know me now," he said.
Mr Mohammed first posted about ISIL on Facebook under his own name, in the first few days after the group's fighters swept into Mosul, but a friend told him he risked being killed. So in those first days he made himself a promise: trust no one, document everything.
A newly minted teacher with a reputation for secular ideas, he had lost his university job.
He found another calling.
"My job as a historian requires an unbiased approach which I am going to adhere to and keep my personal opinion to myself," he wrote on that first day, June 18, 2014.
Mosul Eye became one of the outside world's main sources of news about the ISIL fighters, their atrocities and their transformation of the city into a grotesque shadow of itself.
During Friday sermons, Mr Mohammed feigned enthusiasm. He collected propaganda to post online later. He drank tea at the hospital, fishing for information.
Much of what he collected went on the blog. Other details he kept in his computer, for fear of giving away his identity. Someday, he promised, he would write history with them.
The most sensitive details initially came from two old friends: a doctor and a high school dropout who had joined an ISIL intelligence unit.
Mr Mohammed's information sometimes included photos of the fighters and commanders, complete with biographies surreptitiously pieced together during the course of his normal life — that of an out-of-work scholar living at home.
"I used the two characters, the two personalities to serve each other," he said. He expanded into a Facebook page and a Twitter feed to parcel out information at a time when little news was escaping.
Intelligence agencies made contact as well and he rebuffed them.
"I am not a spy or a journalist," he would say. "I tell them this: If you want the information, it's published and it's public for free. Take it."
But in March 2015, his catalogue of horrors got to him.
"I was super ready to die," Mohammed said. "I was so tired of worrying about myself, my family, my brothers. I am not alive to worry, but I am alive to live this life. I thought: I am done."
He cut his hair short, shaved his beard and pulled on a bright red sweater. His closest friend joined him.
They drove to the banks of the Tigris blasting forbidden music. They shared a carafe of tea. Heedless of people picnicking nearby, Mohammed lit a cigarette — banned by ISIL. Somehow, incredibly, he wasn't caught.
"At that moment I felt like I was given a new life."
He resumed what he had taken to calling his duty. He grew out his hair and beard and put the shortened trousers back on.
He tested out different voices, Christian, Muslim. Sometimes he indicated he was gone, other times that he was still in the city.
Finally, after leaving Mosul a thousand times in his mind, he decided it was time to get out.
"I think I deserve life, deserve to be alive."
A smuggler agreed to sneak him out for US$1,000 (Dh3,672.). Mr Mohammed left the next day, the contents of his computer transferred overnight to a hard-drive that he packed with him.
No one gave him a second look during the two days and some 500 kilometres it took to reach Turkey.
Once there, Mr Mohammed kept Mosul Eye going via WhatsApp and Viber, from Facebook messages and long conversations with friends and relatives who had contacts within ISIL. From hundreds of kilometres away, his life remained consumed by events back home.
By mid-2016, deaths were piling up faster than he could record. ISIL was on a hunt for traitors and the airstrikes were taking an increasing toll on everyone. Mr Mohammed's records grew haphazard, and he turned to Twitter to document the atrocities. In February 2017, he received asylum in Europe.
Only after his elder brother Ahmed was killed in a mortar strike and ISIL was gone from the city did Mr Mohammed reveal his secret to a younger brother — who greeted the news with a shock of pride and happiness. His sibling spoke on condition of anonymity from his refuge in Iraq because he was fearful for his life.
"People in Mosul had lost hope and confidence in politicians, in everything," his brother said. Mosul Eye "managed to show that it's possible to change the situation in the city and bring it back to life".