Three women and a man were fatally stabbed in the London borough of Southwark early Monday, the police said.
A suspect was arrested and is in custody, the police added.
The police say they believe that the attacker and the victims knew each other, but no information was immediately available on the circumstances surrounding the killings.
Officers responded to reports of a disturbance at 1:40 a.m. on Delaford Road in Southwark. After forcing entry, they found four people inside with what appeared to be stab wounds, the police said.
The four people were pronounced dead at the scene, the police added.