A 43-year-old man who was arrested in connection with a murder in Hastings Street was released on bail on Saturday.
The man was arrested at a house following the death of a 19-year-old in the early hours of Friday morning.
The 19-year-old died from a stab wound to the heart, a post mortem confirmed.
A member of the public reported the man had been found at 12.40am, and police officers administered first aid until the ambulance arrived but the man died at the scene.
Det Chief Insp Sharn Basra said: “We are treating this as an isolated incident and some people have already spoken with detectives but we still need to trace and speak with a number of people.
“I am particularly interested in tracing four men who were seen going in to an alley way which runs between two houses that have been converted in to flats in Hastings Street.
“It is believed one of the men then returned to the entrance of the alley way and turned right towards The Butcher’s Public House where he stood and is joined moments later by another member of the group. Both men are then joined by the remaining two group members before they all leave the scene towards Elizabeth Street.
“All the group are described as wearing hooded tops; one is described as black and another is described as a white man with short straight ginger hair which was brushed forward.
“Two other men were also seen on CCTV in the area at around the time of the incident. They are seen to be walking from Hastings Street towards the direction of Windsor Street.
“These people may be vital witnesses to the incident and I need them to contact the police urgently.”
Anyone with information can contact Bedfordshire Police, in confidence, on 101, or text information to 07786 200011. Alternatively you can contact the independent crime fighting charity Crimestoppers, anonymously, on 0800 555 111.