PC Wayne Couzens 48, arrested for at an address in Kent along with a woman 39.
Originally arrested for kidnap and later re-arrested for murder.
The woman was also arrested at the same location on suspicion of assisting an offender.
Both have been put in custody at a London police station where they remain.
The house in Deal, Kent has been cordoned off with metal fencing and currently, the home and garden are being searched. Locals report a police helicopter was searching the Ashford woodland nearby.
Their £20,000 black Seat Exeo was taken away on lowloader.
150 officers have joined the search in Kent, 30 miles from where the police officer and his wife live.
Sarah’s family has been kept updated of these developments.
Sarah spoke to her boyfriend for about 15 minutes while walking and the phone call ended at 9.28pm. Since then, no one heard anything from her and her last known location was near Clarence Avenue.
Detective Chief Inspector Katherine Goodwin said: “This is a significant development in our inquiry. This is a fast moving investigation and we are doing everything we can to find Sarah.
“We have seen an overwhelming response from the public and I repeat my request for anyone with information that may be relevant to come forward, no matter how insignificant it may seem.”
Assistant Commissioner Nick Ephgrave said: “The arrest this evening is a serious and significant development. We will continue to work with all speed on this investigation but the fact that the arrested man is a serving Metropolitan Police officer is both shocking and deeply disturbing.
“I understand there will be significant public concern but it is essential that the investigative team are given the time and space to continue their work.”