Officers smashed their way into the home of a mother-of-two who was found stabbed to death, while the bodies of her husband and children were discovered 145km away.
Six police cars raced to the three-bedroom home in Twickenham, South West London, just after 6pm yesterday and found the 47-year-old woman covered in blood after she suffered stab wounds.
Detectives tried to trace her husband, 57, and two sons, aged 7 and 10, and were contacted by Sussex Police after the bodies of a man and two children were found at Birling Gap near Eastbourne an hour earlier at 4.55pm.
A family car was recovered near the 90m cliff edge in East Sussex and forensic officers from Scotland Yard were at their home address trying to piece together the sequence of events.
No arrests have been made and police said that at this stage they were not looking for anyone else in connection with the investigation.
The London home - which neighbours believe was rented - remained sealed off with police tape and had a big police presence outside. Officers in forensic uniforms were seen inspecting the front garden and bins.
Neighbour Alycia Mead, 25, said: "It was about 6pm and I saw lots of flashing lights and two fire engines parked on the road.
"Then about six police cars, some unmarked, came and the officers were smashing their way through the front door.
"They were struggling but managed to smash through some of the windows and climbed in through the door and then I heard them say 'she is on her own'.
"They were picking their way through the garden and the shed and when I saw forensics last night I thought it must have been murder.
"I haven't really seen them, I saw Christmas lights go up but that is about it, they were a very quiet family."
Paul Warrington, 54, heard nothing from the family on on Monday evening and only heard about the killing when police knocked on his door.
The graphic designer and dad-of-two said he last saw them around 10 days ago and regularly heard the children screaming and shouting as they played in the garden.
He said: "They were a pleasant and quiet family. I often heard the kids playing in the garden, screaming and shouting.
"There was nothing untoward that would suggest anything like this would happen.
"They had lived there for about a year, they were very normal, you never imagined something like this would happen."
A former owner of the home confirmed the house was sold in 2014 and is believed to have been rented out to the family, who are said to be Polish.
Sussex Police said the bodies had not been found in the vehicle and had been discovered by a member of the public. Post-mortem examinations have yet to take place and next of kin have been informed.
A police scene of crime officer spent the afternoon at Birling Gap taking photographs of the spot, where hundreds of sightseers were admiring the scenery at the iconic white cliffs unaware of what had happened.
The remote area of coastline features only a cafe, where the manageress said: "It is just heartbreaking. At 5pm on Monday the police and coastguards arrived and said there had been a major incident along the cliffs.
"But when I found out what had happened I was horrified.
"We then heard about the three bodies on the beach. We all feel terribly sad - this is beautiful place but with Beachy Head so close we do hear some awful news at times. Our hearts go out to this poor family."
A Scotland Yard spokesman said: "A murder investigation has been launched after a woman was found dead at her home address in Twickenham, and her husband and two children died in East Sussex.
"Police were called at approximately 5.55pm on Monday to TW2 following concerns raised about the welfare of the occupants. Officers attended along with London Ambulance Service.
"A crime scene remains in place at the Twickenham address. Officers from the Met's Homicide and Major Crime Command are leading the investigation, supported by colleagues in Sussex Police. "