The collision, which killed two teenagers, sparked a riot in the Ely area of Cardiff on Monday night, with 15 officers being injured.

Kyrees Sullivan, 16, and 15-year-old Harvey Evans died in the crash while riding an electric scooter.

Disorder erupted 'because police would not let parents get close to scene'

The video, lasting a few seconds, shows a bike travelling along Frank Road in Ely at 5.59pm on Monday less than one mile from the suspected crash site on Snowden Road with a marked police van about 15 metres behind it.

The bike is moving at about 28mph, with the van going at a similar speed.

Some residents in the district claimed the teenagers were being pursued by South Wales Police - an allegation denied by the force.

Officers say none of their vehicles were on Snowden Road when the crash happened.

Footage that "shows a police vehicle following a bike at just prior to 6pm" is forming part of their investigation, the force said.

It received a report of a serious collision at 6.03pm on Monday and a police vehicle in Grand Avenue responded, with officers performing CPR, Chief Superintendent Martyn Stone told reporters.

"The investigation has involved studying CCTV and tracking data from the police vehicle," he said

"At this stage, we do not believe that any other vehicle was involved."

An electric bike has been seized from the scene.

South Wales Police has now referred itself to the Independent Office for Police Conduct (IOPC).

The footage was taken at the house where a relative of one of the boys lived.

Read more: Close-knit community in shock after fatal crash followed by violence

Police said the collision had already occurred when officers arrived, and they remained at the scene to manage "large-scale disorder" until early on Tuesday.

During the unrest, fireworks and other missiles were thrown at a line of officers with riot shields who were blocking one end of the street.

Chief Superintendent Stone revealed 11 injured officers went to hospital while four others were treated at the scene.

Harvey's godmother Jenny Sampson said the disorder broke out because police wouldn't let their parents get close to the crash scene.

She said: "Basically we were all at the scene, the police were just having none of it, they wouldn't let the mums, the dads come up and see their own kids laying on the floor.

"They wouldn't let [the parents] do nothing, it was disgusting how they treated them, and they made them walk home and give them the news in the house, didn't give them any sort of news at the scene, we were there for hours waiting and waiting and they still wouldn't let them through to see if their son was OK."

Asked what caused the rioting, Ms Sampson said: "The police, how bad the police treated everybody, you can't treat families like that... With two young kids laying on the floor, and their own family couldn't even get to them, they couldn't even see their own son's last breaths, it's wrong and it's disgusting."

She added: "They were telling everybody to go away and you can't do that... We were all standing there for about two to three hours before anyone had any information. It was really, really bad."

Chief Superintendent Stone told reporters: "I would like to acknowledge the impact last night's disorder had on local residents, who are understandably very frightened."

He vowed to "arrest all of those responsible. A number of arrests have already been made and more will follow".

He appealed for witnesses and said footage can be submitted on its website.