Guests at a party held in Nottingham could have "significant information" about a shooting nearby, police said.
A 24-year-old man is critically ill after being shot in the Westville Gardens area of St Ann's in the early hours of Sunday.
Detectives want to trace anyone who attended the house party, even if they did not witness the attack itself.
The victim, who is from Nottingham, underwent emergency surgery at the city's Queen's Medical Centre.
Det Ch Insp Tony Heydon, who is leading the investigation, said: "We would like to speak to anyone who was at that party who has not yet been in contact with the investigation team.
"Whether they saw the incident taking place or not, they could have significant information that could assist our investigation and help locate the person or persons responsible.
"Although this is an isolated incident, a man is critically ill in hospital and it is vital that anyone who was at the party or in the area around 1am contacts us as a matter of urgency."
A forensic examination of the scene - a paved and grassed area in the centre of Westville Gardens - has been carried out, and police now believe a number of shots were fired, though no-one else is thought to have been injured.
House-to-house enquiries are being carried out and officers are also examining CCTV footage of the area.