Police in the Constance Marten and Mark Gordon investigation believe a baby found in woodland in Brighton “may have been dead for some time”.
Officers have also been unable to confirm the infant’s gender after the remains were found on Wednesday, and a postmortem has not yet taken place.
It is “too early” to provide a specific date of death, the Metropolitan Police said, adding that the infant has been dead for “several weeks”.
Speaking at a press conference on Thursday, Detective Superintendent Lewis Basford also said the case has been referred to the police watchdog.
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He said: “At this stage we have not yet been able to confirm the baby’s gender and postmortem examination has not yet taken place.
“Despite this, based on our inquiries we’ve carried out so far, we believe sadly the baby may have been dead for some time before they were found.
“It is too early for us to provide a more specific date.
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“Because we believe the death occurred during the course of a missing person investigation we have made a mandatory referral to the Independent Office for Police Conduct (IOPC). This is standard protocol for such circumstances.”
The IOPC said in a statement that it is “currently assessing the referral to determine what, if any, further action is required from us”.
Det Supt Basford added that officers investigating the case are “devastated” by its outcome as he confirmed Marten and Gordon remain in custody – with the warrant for their detention extended until Thursday evening.
“We are truly devastated by the outcome and we know this emotion is being felt here in Brighton and across the country today,” he said.
“We know there are still many unanswered questions, and it is important that we give the investigation team the time and space they need to establish more details about the circumstances of this tragic death.”
Chief Supt James Collis of Sussex Police said the investigation has reached a “new phase” and the police presence around Brighton will be scaled back.
He added that officers have been “overwhelmed by kind messages and support” from the public and thanked “the huge number of people who have helped or offered to help in any way that they can”.
Ms Marten (35) and Mr Gordon (48) were arrested on Monday on suspicion of child neglect after a passerby spotted them in Brighton, and they were rearrested on suspicion of gross negligence manslaughter the following day.
The couple had been on the run since January 5th when their car was found abandoned and on fire next to the M61 in Bolton.
The community remained in shock as the probe continued in Brighton on Thursday.
Residents living near a woodland area between Brentwood Crescent, Hutton Road and Burstead Close told the PA news agency they noticed a heavy police presence around the time officers said remains had been found on Wednesday.
Julia (83) whose house overlooks the woodland said: “I was having dinner and my granddaughter said ‘they have found the baby.’
“I looked out of the window and said ‘they must have found the baby at the back’.”
Wellwishers laid flowers, teddy bears and cards at the scene to pay their respects. – PA