The murders of two men could be linked to organised crime, detectives have said.
They also believe the victims, from eastern Europe, may have known each other.
The first attack happened in Barnet, north London. Detectives found a 30-year-old man with stab wounds in the boot of a car near Scratchwood Park at about 8.25pm on Thursday evening. He died a short time later.
An injured 31-year-old man was also discovered at the scene and taken to hospital. He was later arrested on suspicion of murder.
The second murder victim, 35, was found with stab wounds in undergrowth in Hogg Lane, Elstree, on Friday afternoon, and pronounced dead.
The locations are about five miles apart and officers are trying to work out which death happened first.
The two crimes have been "linked" and the investigation is "fast moving", Scotland Yard said.
"Officers are working to establish a timeline of events in what is a complex investigation," it added.
Post-mortem examinations will be carried out on both victims.
Detective Chief Inspector Simon Stancombe said a "number of leads" were being pursued and that "the two victims might have been known to each other".
He added: "While we retain an open mind as to a motive, we cannot rule out whether there might be a potential organised criminal element.
"Police are keen to hear from anyone who may have information around suspicious vehicles being driven erratically in the Barnet/Elstree areas, or any suspicious activities they may have noticed."
The Barnet murder was one of seven in London in seven days.
Anyone with information should call the incident room on 020 8358 0200, or 101, quoting 7486/19 Dec.