The Metropolitan Police officer detained in connection with Sarah Everard's disappearance was arrested on suspicion of murder and kidnap, it has been confirmed.
Police did not initially say what the unnamed Parliamentary and Diplomatic Protection Command officer was arrested for, but gave confirmation on Wednesday afternoon.
He was arrested in Deal, Kent, on Tuesday night, in a development described as "deeply disturbing" by a senior officer.
Investigators believe he was not on duty at the time Everard vanished, and the Met says it has made a referral to the Independent Office for Police Conduct.
The officer, who was initially arrested on suspicion of kidnap on Tuesday then further arrested on suspicion of murder on Wednesday, was also detained over a separate allegation of indecent exposure.
The Parliamentary and Diplomatic Protection Command is responsible for guarding Parliament, Downing Street and embassies, and he was responsible for uniformed patrolling of diplomatic premises.
An unnamed woman in her 30s was arrested on suspicion of assisting an offender.
The confirmation came as police searched a home and woodland in Kent.
Everard has not been seen since the night of 3 March, when she walked home to Brixton, south London, from a friend's house in nearby Clapham.
The 33-year-old marketing executive crossed Clapham Common, which has been searched by police,
She was last seen on footage recorded at about 9.30am on a doorbell camera, walking along the A205 Poynders Road towards Tulse Hill.
Police searched along the road and used sniffer dogs on the nearby Oaklands Estate.
Everard's uncle Nick told Sky News "things are utterly dire" but that the family is "waiting for positive news from the police".
Officers searched a home in Deal, removing cars from the property and taking photos in a garage, while other investigators went through woodland near Great Chart Golf and Leisure in Ashford, Kent.
A neighbour on the same road as the house in Deal said an officer lives there with his “chatty” wife and two children.
"They just seemed like a normal, regular family, there was nothing strange about them at all," she said, adding that police were digging up the garden.
Neighbours said a helicopter was circling above.
Matt Mullan, 29, said: "Last night there was police cars outside all night. I thought this is a bit strange, not normal for round here.
"It was about 8-9pm there was heavy traffic out there and there has been police cars outside all morning."
Matt Housely said: "First thing you think, has someone been murdered?"
Speaking to journalists outside Scotland Yard on Wednesday, Assistant Commissioner Nick Ephgrave said: "Our inquiries suggest that this officer was not on duty at the time of Sarah’s disappearance."
AC Ephgrave did not say if the officer was known to Everard.
"This is a serious and significant development in our search for Sarah and the fact that the man who has been arrested is a serving Metropolitan Police officer is both shocking and deeply disturbing," he told reporters.
He added: "As a father myself of four young women I can only imagine the anguish that Sarah’s family are feeling at this very very difficult time."