Foyle PSNI are investigating how a security staff member was found unconscious at a building site on Longtower Street on February 7.
At this stage it is not known how the man came to be injured therefore police are appealing for anyone who saw suspicious activity in the area between 10.50pm and 11.45pm to contact them.
The man was treated in hospital for a bump to the head.
Anyone with information about the incident is asked to contact police on 0845 600 8000 or a member of your local Neighbourhood Team directly, details can be found on the PSNI website.
Alternatively, if someone would prefer to provide information without giving their details, they can contact the independent charity Crimestoppers and speak to them anonymously on 0800 555 111.