A man is being held on suspicion of killing his estranged wife and mother-in-law in a knife attack.
The suspect has been named locally as Sergei Zolotovsky, 42. An eyewitness described seeing a man standing over the body of Latvian Svetlana Zolotovska holding a kitchen knife. Her throat had been cut.
Michael Herbert was confronted with the scene after hearing a woman screaming near his home in Beckton, east London on Thursday. He ran to Stansfeld Road where he found a man dressed in black standing over the woman's body.
Mr Herbert, 47, said the man threatened him as he chased him across a park and down an alleyway to a house in nearby Leamouth Road.
The house had been set on fire and when riot police forced their way inside they found the victim's mother had also been fatally stabbed.
The suspect was arrested after being discovered in the loft, where he had slashed his neck.
Detectives are preparing to interview Mr Zolotovsky, who is under guard at an east London hospital.
They believe he ambushed his wife after she left home to go to work as a pharmacy technician at the private Wellington Hospital in St John's Wood, north west London.
Colleagues said the couple were in the middle of a divorce and Mr Zolotovsky had moved into another home a short distance from the scene of the attack.
Mrs Zolotovska's mother, aged in her late 60s or early 70s, moved in to the Leamouth Road home three months ago to support her after the couple separated more than a year ago.