A man has died after struggling in the water at Longbridge Lake near Romsey in Hampshire.
Officers have arrested four people on suspicion of murder.
Emergency services were called to the lake following reports of the man getting into trouble within a fishing lake.
Police responded to the incident where a man began struggling within the water at the three-acre Longbridge Lake.
He was rescued and rushed to Southampton General Hospital where he was pronounced dead.
Hampshire Police say they originally arrested three men and a teenager on suspicion of murder but all four were later released.
The death of the man, from Trowbridge, Wilts, is not being treated as suspicious but a 34 year old man from Salisbury, Wilts, who was questioned is being investigated in connection with an assault which allegedly took place during the incident.
A Hampshire police spokesman today said: “We were called at 2.31am on June 26 following reports of a 30-year-old man in difficulty in the water at Longbridge Lake, near Romsey. He later died at Southampton General Hospital.
“The four arrested on suspicion of murder were a 40-year-old man from Devizes, Wilts, a 48-year-old man from Southampton and a 34-year-old man and 16 year old boy, both from Salisbury.
“All four have been released from custody with no further action.
“However the 34-year-old remains under investigation for an assault in connection with the incident.”