Police have refused to say why its officers were following two boys who died in a crash a short time later, sparking a riot in Cardiff.
Harvey Evans, 15, and Kyrees Sullivan, 16, died in the Ely area on Monday.
Deputy Chief Constable Rachel Bacon said police were on Grand Avenue when the crash happened on Snowden Road, about half a mile away.
But she would not answer questions about the CCTV, citing the ongoing police watchdog investigation.
Ms Bacon said no other vehicles were involved in the fatal crash, but would not comment on why police were spotted following the teenagers on CCTV.
She told a press conference: “I want to be as transparent and open as I can with the communities of Ely so they understand what has happened.
“I’ve set out the timeline based on the factual information that we have.
“But the IOPC are conducting an independent investigation on whether any pursuit has taken place so I can’t fully answer your question today.”
Ms Bacon laid out a timeline of events and said the crash, which killed the two best friends, took place half a mile away from the police vehicle.
Harvey and Kyrees’ deaths sparked a riot which saw cars set alight, fireworks thrown at police and 15 officers injured.
Some residents claimed that the boys were being chased by police when they were killed in the crash.
South Wales Police and Crime Commissioner Alun Michael initially said officers had not been chasing the teenagers when they died.
At Wednesday’s press conference, Ms Bacon said South Wales Police was aware of concerns around the timeline of events, including CCTV footage.
She outlined the timeline from when the boys’ bike travelled towards the police vehicle in Frank Road, before turning around at 17:59, to the crash which happened about two minutes later.
“I’ve been really clear that I’ve given you factual and accurate information,” she added when quizzed over whether BBC footage contradicted her timeline of events.
“The situation yesterday morning was still very unclear.
“I’ve explained to you the huge amount of work that has had to be undertaken to get to the point where we are.
“I would have wanted to speak to our communities sooner and I haven’t been able to because we haven’t had that level of information.”
AT THE PRESS CONFERENCE: BBC reporter Nelli Bird
There was a tense atmosphere in the hastily assembled press conference in the canteen at South Wales Police Headquarters in Bridgend.
At times, Deputy Chief Constable Rachel Bacon looked straight at the cameras when answering questions, speaking directly to the community of Ely.
In the end, journalists were only allowed to ask a handful of questions with some batted away due to the upcoming Independent Office for Police Conduct (IOPC) investigation.
The aim of the press conference was to try to give more information and reassure people in Ely that the investigation was being handled well.
But there are clearly many questions remaining for the community – and for the families.