At one time, Luka's own phone number was hard to avoid - in the early days of the Syrian civil war, it had been spray-painted on walls around the central city of Homs. The conflict was sparked by Assad's brutal crushing of a pro-democracy uprising in 2011 - a war in which hundreds of thousands of people would go on to be killed.

Luka was a local head of the Political Security Directorate, one of Assad's main intelligence agencies, and had been given the task of wresting the city from the rebels. At the time, rebel-held areas in Homs were under siege by the government, and residents were struggling to access food and medicine.

Several people, including former rebel fighter Motaz al-Zaydi, told us they had resorted to eating rats. Al-Zaydi remembers Luka's name and phone number appearing on a wall in his neighbourhood, and said the message began to spread that those wanting to flee the fighting could phone the number for safe passage.

Some are believed to have been granted safe transit out of the city by Luka, but others who called the number were never seen or heard from again, according to several people we spoke to, including al-Zaydi. Many told us they referred to Luka as " The Spider" because of his vast web of allies and informants, including some embedded in rebel groups.

" He was everywhere," Homs residents told us. But his most egregious action in the city, many argue, was his alleged involvement in the so-called Eid Massacre, known by some as the Children's Massacre, which the US noted when sanctioning him."

They were tiny children," he said with tears in his eyes. " The ground here was painted red with blood."

Twenty-eight people were killed, including 17 children and four women, according to the Syrian Network for Human Rights. Luka, according to a Syrian activist group, and a pro-opposition newspaper at the time, was responsible for the regime's selection of targets in the area that day.

His actions in Homs earned him a promotion, and by the time of the regime's collapse he ran the General Security Directorate (GSD) intelligence agency, making him one of the most powerful people in the country. The government security services could target anyone - from opposition fighters, journalists, doctors, or rights activists, to people believed to be opposed in any form to the Assad government.

Those arrested were detained in Syria's political prisons, including Sednaya in Damascus - a facility so notorious it was dubbed the "human slaughterhouse". Many imprisoned there were never seen again.