Three people were stabbed after a fight broke out at a dance music concert.
The incident happened during the Swedish House Mafia show at the Milton Keynes Bowl.
According to Thames Valley Police, one of the victims, a 23-year-old man from the West Midlands area, remains in Milton Keynes General Hospital with a punctured lung.
Another, a 24-year-old man, also from the West Midlands, received a cut to his head and arm but has been discharged from hospital, and a 23-year-old man was treated for minor injuries by St John Ambulance crew.
Detective Chief Inspector Justin Fletcher said: "This appears to have been an isolated incident, and overall we are very pleased with how the event passed off, but we would like to trace the person responsible for this assault."
Police are keen to speak to a man who is described as white, aged in his early 20s, wearing a blue Nike T-shirt, a black jacket and black jogging bottoms, in connection with the assault.
A police spokesman said: "Anyone who witnessed the incident, particularly if they captured it on their camera or mobile phone, is requested to contact the police."
During the concert, three people were also arrested on suspicion of possession with intent to supply class-A drugs.
They are a 19-year-old man from Liverpool, a 26-year-old man from London, and a 37-year-old man from Ipswich.