A man was fatally stabbed during a fight after a three-car crash in the early hours.
The victim, in his 20s, was found with stab wounds just before 03:00 BST in Gammons Lane, Watford.
He was taken to hospital where he was pronounced dead. Police were also called to reports another man had been found nearby with a stab wound.
A man, aged 19 from Watford, has been arrested on suspicion of murder, Hertfordshire police said.
Officers have said the two stabbings were linked and both scenes were sealed off.
The second man was discovered in St George’s Road with a “serious” injury to his arm and taken to hospital.
Det Ch Insp Justine Jenkins, from the Bedfordshire, Cambridgeshire and Hertfordshire Major Crime Unit, said: “We are still in the very early stages of our investigation and extensive inquiries are being carried out to help establish the sequence of events.”
Watford Ch Insp Matt Phillips said he was aware the death would “alarm” the community.
“We are working closely with MP Richard Harrington and Watford Borough Council to support anyone who has been affected by this terrible loss of life,” he said.
The three cars – a grey Volkswagen Golf, a blue Vauxhall Corsa and a black Vauxhall Astra – were all badly damaged in the crash.
Police have asked witnesses or those with dashcam footage of the vehicles in the hours before the crash to contact them.