Police searching for missing Sarah Everard have discovered human remains in woodland in Ashford, Kent.
A serving Metropolitan Police officer was arrested on Tuesday night on suspicion of kidnap and later of murder.
On the day of Ms Everard's disappearance, he was reportedly working a 2pm to 8pm relief shift at the US Embassy in Nine Elms, southwest London, around three miles from where she was last seen.
According to unconfirmed reports, detectives are investigating whether he might have used his warrant card to entice Ms Everard towards his car before snatching her.
Prime Minister Boris Johnson has tweeted: "I am shocked and deeply saddened by the developments in the Sarah Everard investigation. Like the whole country my thoughts are with her family and friends.
"We must work fast to find all the answers to this horrifying crime."
Met Commissioner Cressida Dick told a news briefing on Wednesday: "This evening detectives and search teams investigating Sarah's disappearance have found, very sadly, what appears to be human remains in an area of woodland in Ashford in Kent."
She said the arrest of one of her officers had "sent shockwaves and anger" and that the entire force is "utterly appalled at this dreadful news".
"Sarah's disappearance in these awful and wicked circumstances are, I know, every family's worst nightmare," she continued.
Investigators have not been able to confirm the victim's identity and this "may take some considerable time", she added, but specialist officers are assisting Ms Everard's family.
Commissioner Dick sought to reassure the public that "it is incredibly rare for a woman to be abducted from our streets", but that people, and particularly women, may be feeling understandably "worried and scared".
She said there would be a greater police presence in both Kent and London, where searches have been going on "around the clock".
"We are deeply grateful to all those many, many people who came forward with information that has assisted us," she said after the Met received more than 120 calls and were able to search over 750 homes as part of the investigation.
"My thoughts tonight are with Sarah, her family and her loved ones," she told the briefing at New Scotland Yard.
Steve Lewis, headteacher at Ms Everard's secondary school Fulford School in York, said: "It was with great sadness that we learnt the devastating news of Sarah's disappearance and the events of the last few days.
"Sarah was a vibrant, caring and much valued member of our school community. She left Fulford School in 2005. She is still lovingly remembered by those who taught and knew her. Her joy, intelligence and positive spirit shone within the school. She was a lovely pupil and friend.
"Her family and friends are part of our community and our thoughts and warmest wishes go to all who have been lucky enough to have had her in their lives."
Professor Stuart Corbridge, Vice-Chancellor of Durham University where Ms Everard was a student, said: "We have been made aware of the situation and our thoughts are with the family and friends of Sarah at this difficult time.
"We have contacted the Metropolitan Police and offered our assistance should it be required."
The arrested officer, who was not on duty at the time of Ms Everard's disappearance, is also being held on a separate allegation of indecent exposure.
He was arrested at an address in Deal, Kent, more than 60 miles away from where Ms Everard was last seen.
A woman was taken into custody from the same address and is being held at a London police station on suspicion of assisting an offender.
Extensive searches have been carried out at the house, around Clapham and Brixton and the woodland in Ashford.
A significant police presence remains at a site near Ashford and a wide perimeter has been set up around the scene with officers at the entry points.
On 3 March, Ms Everard left a friend's property in Leathwaite Road, Clapham, around 9pm.
The marketing executive then spoke to her partner on the phone for about 15 minutes, with the call ending at 9.28pm.
The last known sighting of her in Clapham was captured on a doorbell camera.
It showed her walking alone along the A205 Poynders Road from the junction at Cavendish Road, toward Tulse Hill, which lies south of Brixton, at about 9.30pm. She had not been seen or heard from since.