Aristocrat Constance Marten and her partner Mark Gordon have been further arrested on suspicion of gross negligence manslaughter amid a huge search to find their baby.
Metropolitan Police said the couple were located and arrested by Sussex Police in Stanmer Villas, Brighton on Monday night after a member of the public called with a sighting shortly before 9.30pm.
Detective Superintendent Lewis Basford confirmed on Tuesday that the pair had been further arrested on suspicion of gross negligence manslaughter.
He said: “I can confirm that they were initially arrested on suspicion of child neglect.
“I can now confirm that they have been further arrested on suspicion of gross negligence manslaughter and that they remain in custody at police stations in Sussex.
“The baby was not with them, and we have not found the baby as yet.”
Since Marten, 38, and Gordon, 48, were arrested, more than 200 police officers have been engaged in the search for the two-month-old baby between Newhaven and Brighton.
The infant has not received any medical care since birth in January.
The couple have been travelling around the UK by taxi since their car was found burning on the M61 in Bolton, Greater Manchester, on January 5.
Authorities previously believed the couple had been sleeping rough in a blue tent, and had avoided being traced by the police by moving around frequently and keeping their faces covered in CCTV images.
Asked if the couple could have given the baby to somebody else, Det Supt Basford said: “Certainly our main line of hypothesis is that they (Marten and Gordon) were in control and custody of the baby.
“At no point have we seen, in that short period of CCTV… that they allowed others to have that baby.
“In fact that baby was very much concealed within the coat lining of Constance or in the buggy for the short period they had it.”
He added officers “could not rule out” that the couple had found someone “like-minded or who does not conform or appreciate the establishment” and used the money they had for “safe lodging”.
He added that after the arrests, this line of inquiry did not become “as significant”.
Marten’s father, Napier Marten, said it is a huge relief that his daughter has been found, but it is “very alarming” that her baby remains missing.
He told The Independent: “It is an immense relief to know my beloved daughter Constance has been found, tempered by the very alarming news her baby has yet to be found.
“For whatever reasons she and her partner went on the run, the consequences of their actions have increased many fold. It would have been far better if they had handed themselves in earlier.
“When the time comes, I am longing to see Constance to reassure her that, whatever the weather, I love her dearly and will support her as best I can through the difficult weeks and months here on in.”
Marten, who is from a wealthy aristocratic family, was a promising drama student when she met Gordon in 2016.
Since then the couple have led an isolated life, and in September, as Marten’s pregnancy progressed, began moving around rental flats.
Gordon served 20 years in prison in the US for rape and battery committed when he was 14.
It is unknown if their baby was full-term or has any health issues.
In an appeal a week ago, Shereen Nimmo, director of midwifery for Barts Health NHS Trust, urged the couple to bring the baby in for checks.
She said: “You’re putting your baby at risk by not accessing medical care, so it’s really important that you come and see a midwife, doctor or another healthcare professional as soon as possible.
“Babies need a safe, warm environment and all new mums are given information about safe sleeping for their baby to prevent sudden infant death syndrome.
“Please do the right thing for your baby and go to your nearest healthcare facility so my caring colleagues can take care of you and your baby. All we want to do is help you.”
Anyone who may be able to assist in the search for the couple’s baby is urged to contact 999.