When a 68-year-old man entered Lake Tuggeranong and later died last week, this young man was seen running under the Soward Way Bridge nearby.
Police have released a facefit image of a man they think may have important information about what happened to the dead man on December 6.
ACT Fire and Rescue pulled the unconscious 68-year-old from the lake and ambulance officers took him to Canberra Hospital, where he later died.
Police have since revealed the dead man's wallet was missing but his bank accounts have not been accessed inappropriately.
A witness saw a young man aged in his late teens running under the bridge about the time the older man was thought to have entered the lake.
He ran past the witness' car and up towards the stairs onto Soward Way.
He was described as about 165 centimetres tall and Caucasian in appearance, with short messy brown hair, wearing a peach-coloured top, black shorts and dark-coloured shoes.
Police are urging the young man to contact them, as he might have been the last person to see the 68-year-old before he died.
The victim was seen at the lake's edge about 12.45pm.
Crime Stoppers on 1800 333 000, or www.act.crimestoppers.com.au