Constance Marten and Mark Gordon were arrested on suspicion of gross negligence manslaughter.
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Police fear a baby at the centre of a massive search operation has come to “serious harm”.
Hundreds of officers using sniffer dogs, drones and thermal cameras are scouring woodland in Brighton.
Police have asked for more time to question Constance Marten and Mark Gordon who were arrested on suspicion of gross negligence manslaughter.
They were reported missing when their car was found alight near Bolton, triggering a 53 day search.
Police say Ms Marten had recently given birth and believe the baby may have been born in the back of the vehicle, without a midwife or medical attention.
Ms Marten, 35, and Gordon – a convicted rapist and registered sex offender – were arrested on Monday after a member of the public spotted them in a shop and alerted police.
The couple remain in custody, but police said they had not provided any further information about the condition or whereabouts of the child.
Police do not know the sex of the baby.
They were initially arrested on suspicion of child neglect but were re-arrested on Tuesday on suspicion of gross negligence manslaughter.
Speaking at a press conference on Wednesday afternoon, Det Supt Lewis Basford from the Metropolitan Police said a huge search had continued throughout the night.
Image source, PA Media
Members of the public have been asked to “remain vigilant” and report any unusual activity or “suspicious items” they may have seen in recent weeks.
Inquiries are also being carried out in the South Downs and in Newhaven, where the couple were spotted with their baby on January 8, the last confirmed sighting prior to the arrest.
It is believed they were sleeping rough outdoors and were previously spotted carrying a blue tent.
Teams were seen working in Moulsecoomb Wild Park, around a mile from Stanmer Villas in Brighton where the couple were arrested, earlier on Wednesday.
The officers searched under sticks and logs close to where the area meets Hollingbury Golf Course.
Volunteers from London Search and Rescue have been drafted in to bolster efforts.
Allotments and the golf course were searched on Tuesday. A Facebook Group for the Roedale Valley allotments were told police had broken into every shed on the site in their search for the baby.
A car belonging to Ms Marten and Gordon, 48, was found on fire on the side of the M61 motorway in Bolton on 5 January.
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Police said the family left the scene and travelled to Liverpool, Essex, London and East Sussex.
They appeared to cover their faces when in public, travelling at night and using cash to purchase supplies, apparently attempting to avoid detection.
Their home is in Eltham, in south-east London, but they have been living nomadically since September last year when Ms Marten first started to show signs of pregnancy.
Police have not ruled out that someone could be looking after the baby – though they added this was unlikely.
Det Supt Lewis Basford said the risk to the baby – who is now thought to be about two months old – increased as time went on, particularly while the weather is cold.
He said on Tuesday that police must now “be open to the fact this may not end in the way we would like”.
Image source, GMP
Image source, PA Media
Police previously said Ms Marten’s inherited wealth may have allowed the couple to remain at large for an extended period.
She is from a privileged background, having lived in a stately home growing up.
She became estranged from her family after meeting 48-year-old Gordon at drama school in 2016.
They believe the baby was alive at the time of the last previous sighting of the family in Newhaven, but little more is known about the family’s movements in the weeks since.
After the pair were found on Monday, Ms Marten’s estranged father Napier Marten told the Independent he felt “immense relief”, though this was “tempered by the very alarming news [her] baby has yet to be found”
People charged with gross negligence manslaughter could face up to 18 years in prison if found guilty over a death caused by negligent behaviour.
This is different to unlawful act manslaughter, where a prison term can be up to 24 years if someone is proven to have intentionally done something unlawful or dangerous that inadvertently caused death.
