Essex police said detectives investigating the murder of Jay Whiston have set up a dedicated number for witnesses to call.
Anyone with information is asked to contact the incident room on 01245 452882.
Up to 200 people attended the house party in Colchester were the murder took place. For more on this story go to ITV Anglia.
More top news
The child died on her injuries and another 14-year-old girl was injured after the buggy they were travelling in was involved in a car crash.
Cloud thickening from the west throughout the day. Rain in northwest later.
The Songs of Praise presenter has voluntarily agreed not to appear on BBC whilst an investigation takes place.