A teenage boy was stabbed to death and two other family members injured in the attack, police have said.
The 17-year-old was killed at a property in Ninian Road, Cardiff, South Wales Police said.
The force is trying to trace two men seen leaving the area shortly after the incident and heading towards Shirley Road, Cardiff.
One of the men is described as being of Asian appearance, aged late 20s, of average height and stocky build, with dark hair and wearing dark clothing.
The second man is described as being of Asian appearance, aged early 20s, of slim build, and 5ft 11ins tall. He was wearing a white jacket with a grey stripe down the sleeves.
A major incident room has been set up at Cardiff Central Police Station.
Chief Superintendent Bob Tooby, divisional commander of Cardiff Police, said: "This is a very serious incident and I wish to reassure the local community that a full investigation is under way in order to trace and arrest those responsible.
"Our thoughts are with the family of the victim, who will be supported by a trained family liaison officer at this most difficult time.
"I would urge anyone with information about this incident to come forward and speak to the police immediately."
Witnesses or anyone with information have been asked to contact the incident room at Cardiff Central Police Station on 02920 571530 or Crimestoppers anonymously on 0800 555 111.