Police are searching for a knifeman after four people were stabbed in a spate of attacks in north London over the weekend.
A woman and three men were all approached from behind and knifed in the back as they walked alone in Edmonton.
Police said each victim appeared to “selected at random” for being “alone and vulnerable” in the potentially linked attacks.
Two are in a critical condition.
The Metropolitan Police said officers were working on the theory the suspect acted alone and “mental health issues may be a factor”.
A man has been arrested on suspicion of causing GBH but police say inquiries continue to establish if he is the person behind the stabbings.
The first attack was on a 45-year-old woman who was hurt in Aberdeen Road at 19:02 GMT on Saturday. She remains in a critical condition in an east London hospital.
Four hours later, a man was stabbed half a mile away in Park Avenue. His injuries are not thought to be life-threatening.
Police believe the third attack happened less than a mile away in Silver Street.
The victim, a 23-year-old man, was found injured at Seven Sisters Tube station at about 04:00 BST. He is in a critical condition in hospital.
The final stabbing happened at 09:42 in Brettenham Road, less than a mile away from Silver Street. Police say a man has been taken to hospital with potentially life-changing injuries.
Detectives said they were searching for a slim black man, who is about 6ft 3in tall and was wearing dark clothing.
Det Ch Insp Stuart Smillie said: “Police are treating the incidents as potentially linked. The four victims are all from different backgrounds and appear to have been selected at random due to them being alone and vulnerable.
“He has approached from behind without warning.”
Extra officers are on the streets to keep the public safe, a spokesman said.