This was “coordinated” with a prison uprising, where inmates seized weapons and captured guards, Al Naba said. IS footage from inside the prison showed cheering fighters wearing red headbands, signifying they were ready to fight to the death.

Other footage showed chaotic scenes as the prisoners overran the prison, seizing guards and staff as hostages. In one clip, fires burn in the prison yard as a pick-up truck repeatedly reverses into a wall to break through it, as men shout Allahu Akbar.

The group’s Amaq news agency initially claimed that 800 prisoners had escaped and that the remaining prisoners would continue fighting until they were free or die trying. But the prison break soon turned into a siege as the SDF locked down the surrounding neighbourhoods and called upon British and American special forces and coalition air strikes for support.

IS accounts were soon sharing grisly videos showing injured prisoners groaning amid piles of bloody corpses.

“There’s a lot of injured people here screaming in pain,” said a 17-year-old Australian inside the prison who sent voice-notes to his family in Sydney.

“I lost a lot of blood, I don’t know what to do, there’s no doctors here,” said the teenager, who said he had been injured by munitions fired from an Apache helicopter. He was just one of at least 700 minors held in the prison for suspected family links to IS or on suspicion of having been a child fighter.

With food and water supplies cut off, by Monday hundreds of prisoners were surrendering, with SDF footage showing crippled men hobbling in single file to be searched, many bearing apparently old injuries.

‘Poor security and overcrowding’

SDF fighters then cleared the facility one building at a time. A former school, Al-Sinaa was never intended to hold thousands of dangerous prisoners. A single high concrete wall is all that separates the compound from an adjacent public highway.

While the SDF’s prisons were never meant to be a permanent solution to the problem of IS members, over time the global coalition has begun to upgrade the facilities.

Concerned by poor security and overcrowding, in which communicable diseases such as tuberculosis are said to be epidemic, the UK alone has provided millions of dollars in funding.

“Our British coalition partners have invested 20 million dollars in expanding the Hasaka prison. We anticipate that the project will be completed in September,” then-coalition commander Lieutenant General Paul Calvert said last August.

But SDF footage suggests that a new extension was not yet in use holding prisoners. In one clip, female fighters clearing buildings dash into newly painted cells, empty except for piles of plastic wrapped mattresses.

The SDF says some 3,500 prisoners surrendered, who must now somehow be reshoused in other northeast Syrian facilities. Footage suggests that Al-Sinaa prison has sustained heavy damage, including from coalition air strikes.

Despite the money spent on upgrades, the ease with which the militants overran the facility suggests systemic security failures.

‘Prisoners had access to mobile phones’

Vera Mironova, a non-resident fellow at the Middle East Institute who monitors and speaks with IS supporters, said prisoners in Al-Sinaa had access to clandestine mobile phones that could have only been smuggled in by corrupt guards.

This communication with the outside world was “critical” to the success of the attack, she told The Telegraph.

“Why was this prison in such bad shape?” she asked. “How much money and time do you need to build a legit prison so it can hold?”

As the SDF continued clearing the facility, on Friday they found the bodies of 18 of its members who had been killed in the attack.

While the final death toll is unclear, war monitor the Syrian Observatory for Human Rights say that 73 SDF fighters had died, alongside seven civilians and 180 prisoners and IS fighters.

Attack could boost IS fundraising

IS supporters are hailing this as a major victory. “The morale right now is just unbelievable,” said Ms Mironova.

In Al-Hol camp, the nearby internment camp holding some 60,000 mostly women and children affiliated with IS, the group’s supporters had announced a three-day fast in solidarity with their menfolk, Ms Mironva said.

Online, huge numbers of pro-IS accounts have reappeared across various social media channels in numbers not seen in years. IS propaganda videos remain easily found on Twitter, a week after they were posted.

Beyond the propaganda value, the attack will likely provide a boost to IS fundraising efforts.

“I’m sure now you’ll see a lot of money being sent to those people because it’s increased the propaganda [available] to their fans abroad,” said Ms Mironova.

As the last militants remain holed in the prison, the head of the anti-IS coalition renewed calls for countries to coordinate a more sustainable solution to IS prisoners.

“The makeshift prisons throughout Syria are a breeding ground for Daesh’s failed ideology,” said Maj Gen John Brennan, commander of the anti-IS coalition, referring to IS by its Arabic acronym. “We must thoroughly investigate the circumstances that allowed this attack to happen.”