Police investigating the murder of a 12-year-old girl in Liverpool city centre are going to review hundreds of hours of CCTV in an attempt to find out what happened.
Ava White suffered "catastrophic injuries" when she was assaulted after a Christmas lights switch-on event on Thursday evening.
Merseyside Police said she had been involved in a "verbal argument" that escalated into "an assault on her with a knife", but a weapon has not yet been recovered.
The attackers ran off up School Lane, across Hanover Street and into Fleet Street.
Members of the public who were in the city centre are being urged to check their phones for photos and footage that might assist police in their investigation.
Four boys - one aged 13, two aged 14 and one aged 15 - remain in custody after being arrested on suspicion of murder.
Ava has been described as a popular and bright girl - a "bubbly character" who attended Notre Dame Catholic College.
In 2019, she won a Peace Ambassador award from the Peace Foundation, which was set up by Colin and Wendy Parry after their son Tim was killed by an IRA bomb in Warrington.
Flowers have been laid on Church Street - not far from where the attack happened - which is one of the busiest areas for shopping in Liverpool.
A 17-year-old who left a balloon at the scene described her as "loving and caring" - and said her sister had been close friends with Ava.
She added: "She came out with her friends to enjoy herself and I think it's just wrong that this has happened."
Her headteacher, Peter Duffy, described her as "an incredibly popular girl with a fantastic group of friends".
Assistant Chief Constable Ngaire Waine said Ava's death was "sad for the city" and "even sadder" for her loved ones.
She added: "We will put everything we can to ensure we get justice for her but then I'm sure we will be looking at whether there were things we could have done, or our partners could have done, that might have made a difference, because we never want another 12-year-old, here or anywhere else, to be murdered when they are out and about having fun with their friends."
Anyone with footage can send it to police via https://mipp.police.uk//operation/05MP19M36-PO1.