He died in the early hours of Saturday, June 1, after he was trapped inside a stolen red Mazda 6 car that caught fire after it struck a lamp post on Fairview Road in Galliagh.
The community in Creggan was subjected to high-level anti-social behaviour sparked by joyriding youths in two cars in front of St Mary’s Church where Requiem Mass for Mr Cassidy had taken place at lunchtime on Friday.
During the unfolding chaos, a teenage male was discovered by a police patrol lying in grass shrubland and was taken to hospital where he was treated for puncture wounds to his shins.
Police believe violent dissident republicans carried out the vicious attack.
However, senior officers would not be drawn on local speculation that the victim was attacked with a pick axe, describing his injuries as “not life threatening”.
There was also a fracas near the St Mary’s Church after the teenager’s funeral.
Three windows at a house in Culdaff Gardens in Creggan were also smashed in the early hours of Saturday morning, which is also understood to be connected to the aftermath of Friday’s mayhem.
On Friday schools and youth groups cancelled events and businesses shut early with some saying they felt the community was being held to “ransom”.
Community representatives said police had questions to answer over their response.
“The police can’t win in this situation,” Creggan priest Father Joe Gormley told the BBC.
He said dissident republicans saw themselves as “self-appointed” protectors of the community and the PSNI need to work with the community to “normalise policing”.
“They are damned if they do and damned if they don’t. If they were to come in to the area some of those militant republicans would have easily orchestrated a riot,” he said.
“They have manipulated people’s minds to set themselves up as saviours of the people, which they are not.
“They have their own narrow agenda. Do we want vigilante policing or do we want normal accountable policing?”
PSNI Chief Inspector Johnny Hunter said police were on the outskirts of the Creggan and had carried out “discrete patrols” admitting police presence was increased due to concerns raised prior to the funeral.
“We were in the position of do we put officers in the Creggan or do we respond to incidents as and when they happen?”
He said responses had to be carefully judged and measured saying those in the car may have been looking for a police chase in a residential area as opposed to his officers waiting outside to try and stop it. Although they did not manage to stop the vehicle or capture those inside.
Chief Inspector Hunter said police would be reviewing their response and what lessons can be learned. He said he understood the community’s concerns. He said police observed a group in the area and without intervening the crowd dissipated without “any other major issues later in the evening”.
“It is very difficult for police to balance the consequences of us taking action which at the time can lead to more serious offences especially around stolen vehicles or vehicles being driven erratically,” he said.
“And we have to be very, very careful around our various options.”
He disagreed with a community perception there had been a change in police approach to policing in the Creggan following the murder of the journalist Lyra McKee. He said police teams had “good relations” with those in the area.
“We face challenges… but we are doing our best to get into the Creggan and provide a normalised policing response,” he said.
“We don’t get it right on every occasion and there are always lessons to be learned.
“We have strong relationships, we will listen to feedback and use that as part of this review into this incident.”