Officers said they were examining the "culpability" of "all parties" involved in the death of Elizabeth Cooke on Boxing Day.

The youngster and her 10-year-old brother Jack were riding petrol-engined quad bikes behind their father Gary's Range Rover near the family home in Blackmore, Essex when the accident happened.

Elizabeth, who had been given the quad bike for Christmas, was initially taken home and died in hospital later after suffering "internal" injuries.

Police said Elizabeth's quad bike was in collision with a Range Rover driven by a 28-year-old woman from Brentwood, Essex, shortly after 6pm.

A spokeswoman said officers were continuing to investigate the accident and added: "We are making further inquiries and examining the culpability of all involved parties."

Sources said the investigation could take months.

The 28-year-old woman, who has not been named, was initially arrested on suspicion of causing death by careless driving while unfit through drink or drugs.

Police today said she would not be charged with that offence.

But officers said the woman could be charged with careless driving.