Ridwaan Shaikh, 22, hit Bradley Cresswell, 21, after they had been to Hydeout Nightclub in Watford on 17 September.
Shaikh was jailed for nine years for manslaughter on Thursday.
The Independent Police Complaints Commission (IPCC) is now probing Mr Creswell's contact with Hertfordshire Police shortly before he was killed.
Mr Creswell and Shaikh, of Ilford in London, were with groups of friends when they encountered each other at the nightclub.
Shortly before closing time, one of Mr Cresswell's friends knocked one of Shaikh's friends to the floor.
Mr Cresswell later separated from the group and was walking alone towards Bentine Lane when Shaikh knocked him unconscious, although he soon recovered.
The two groups met each other again in The Parade about 40 minutes later and began arguing, and Shaikh hit Mr Cresswell, this time fatally. Mr Cresswell suffered a bleed to the brain and never regained consciousness.
The IPCC says Mr Cresswell spoke with police officers after the first assault.
An IPCC spokesman said: "A notice of investigation has been served on one officer.
"Misconduct notices do not imply guilt but are to inform the officer that their behaviour and conduct is under investigation and the level of severity.
"The investigation had been suspended while parallel criminal proceedings took place, and will now be resumed."
Shaikh had denied manslaughter but was found guilty at the Old Bailey on Wednesday.
A police officer is under investigation after a man was punched and killed during a night out.