Four men, aged 15 to 18, and a 15-year-old girl have been charged and appeared in court in connection with the incident in Mabgate shortly after midnight on Sunday, March 31, in which a 52-year-old man and a 47-year-old woman were stabbed and seriously injured.
Detectives now want to trace a man who was seen being hit in nearby Regent Street a short time before.
A spokesman for the force said: "Another man reported seeing the unidentified victim being punched in the face near to the A64 flyover sometime after 11.40pm on Saturday, March 30.
"The victim, who ran off under the flyover, was described as being white, aged about 23 years and of big build.
"He was wearing black trousers and a black long-sleeved top.
"The witness, who was also approached and punched in the face as he stood nearby, spoke to the other victim near to the flyover shortly afterwards."
Detective Inspector Phil Jackson, of Leeds District CID, said: “We are keen to trace the man who was seen being punched to speak to him about the circumstances of the assault and because he may have information that could assist the wider investigation.
“We would also like to speak to anyone who was in the area and witnessed any part of these incidents.”
The unidentified victim or anyone who has any information that could assist the investigation is asked to contact DC 959 Alexa Worsnop of Leeds District CID on 0113 3859405 or via 101 quoting crime reference 13190165569.