An elderly man and a middle-aged woman have died after they were stabbed at a sheltered housing complex.
A man in his mid-80s and a woman in her mid-50s were attacked at the Fernways complex in Ilford, east London, on Saturday night. They died at the scene.
A 50-year-old man was arrested at the home on suspicion of murder and taken to an east London police station, where he remains in custody.
Both of the victims and the man arrested are believed to have known each other, but they were not related, a Scotland Yard spokesman said, adding that none of the three was a member of staff at the complex.
Police, London Ambulance Service and London's Air Ambulance were called to the sheltered accommodation unit on Cecil Road at 10pm.
The two victims were discovered with stab injuries. The woman was pronounced dead at 10.50pm and the pensioner was declared dead 15 minutes later.
Next of kin have been informed but formal identification has not taken place. Post-mortem examinations will be held this afternoon at East Ham Mortuary.
Fernways Sheltered Housing unit is managed by Redbridge council which provides 24-hour on-site care to tenants.
Krishna Sheth-Gilston said her aunt, who lives in the accommodation, heard the commotion in the flat above her and feared the floor would fall in.
"There was a lot of noise coming from the flat above her. At first she did not think much of it, but then it carried on," Ms Sheth-Gilston said.
"There was lots of screaming and fighting."
Her aunt notified the warden after becoming "nervous" about "thumping" sounds.
"She thought the floor was going to cave in," Ms Sheth-Gilston said.
She said that the accommodation was open to visitors, but she felt security was good.
"I do not think there are any restrictions," she said. "You have to call in. You cannot just walk in. It's really good security."
"They are all very well looked after", she added.
Detectives from the Homicide and Major Crime Command are investigating the double murder.
Officers remain at the scene and are speaking with residents to provide reassurance.
Detectives are not seeking anyone else in connection with the investigation.
Anyone with information is asked to contact police on 020 8721 4868 or Crimestoppers anonymously on 0800 555 111.