South Wales Police has referred itself to the policing watchdog as CCTV footage showed a police van following two people on an electric bike before a fatal crash which sparked a riot in Cardiff.

Serious disorder broke out in the Ely area of the Welsh capital last night after the deaths of two boys, Kyrees Sullivan, 16, and 15-year-old Harvey Evans, who were riding an electric bike.

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

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

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.

They added that they have received footage that “shows a police vehicle following a bike just prior to 6pm” and the video is forming part of their investigation.

The force 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.

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.

The unrest erupted after the fatal road collision, with fireworks and other missiles thrown at a line of officers with riot shields who were blocking one end of the street.

At a news conference, Chief Superintendent Martyn Stone revealed that 15 officers needed medical attention following the disorder. Eleven went to hospital and four were treated at the scene.

Harvey’s godmother Jenny Sampson, who confirmed the identities of the two boys to Sky News, 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”.

“South Wales Police has also made a mandatory referral to the Independent Office for Police Conduct to ensure the matter receives independent scrutiny.”

He appealed for witnesses, information and footage, and said details on how to submit these can be found on its website.