Residents in north London where four people were “randomly” stabbed within 10 hours have been told to be vigilant despite two arrests being made.
Two people are still in a critical condition in hospital following the attacks in Edmonton over the weekend.
Det Supt Luke Marks said people should “remain vigilant, especially when walking alone”.
A nearby grammar school has put a “lock-down procedure” in place as a “precautionary” measure.
In an email to parents, head teacher of the Latymer School Maureen Cobbett said students would not be allowed to leave the school during breaks and lunchtime, and after-school activities would not take place “until further notice”. Police believe the four people attacked, who were all stabbed in the back, had been selected randomly.
The first victim, a 45-year-old woman, remains in a critical condition after she was attacked in Aberdeen Road at 19:02 GMT on Saturday.
Four hours later, a 52-year-old man was stabbed half a mile away in Park Avenue. He has since been discharged from hospital.
The next victim, a 23-year-old man, is in a critical condition after being attacked near Seven Sisters Tube station at about 04:00 BST on Sunday.
The final stabbing happened at 09:42 in Brettenham Road where a 29-year-old man was attacked. He is receiving medical treatment but his injuries are not life-threatening. Two men are in custody after being arrested on suspicion of causing grievous bodily harm.
One was held in Fore Street, Edmonton, on Sunday morning while a man aged in his 40s was arrested at a residential address at about 19:00.
Det Supt Marks said police still “believe that a single suspect was involved” and inquiries were ongoing, although the attacks were not thought to be terror-related. The suspect has been described as a slim black man, who is about 6ft 3in tall and was wearing dark clothing.
The Met previously said they thought “mental health issues may be a factor” and extra police patrols had been put in place “to keep the public safe and catch the individual responsible”.