Officers attended a property in Walliker Street, Hull, at around 3pm on May 19, following a concern for safety for the occupant.
Mr McManus, 40, was sadly found dead inside the house.
Detective Chief Inspector Alan Curtis said: "Officers are investigating into Lee’s death which is currently being treated as unexplained and his family are being assisted by specially trained officers through this difficult time as they deal with the loss of a loved one.
"We are appealing for anyone who may have seen Lee or interacted with him on or after Tuesday, May 11, to get in touch.
“Your information could be vital in helping us trace the last movements of Lee.
“I want to thank those of you who have assisted with our enquiries so far and have come forward to provide information at this time."
Anyone with information can call Humberside Police on 101 quoting log 322 of May 19, or alternatively, call Crimestoppers anonymously on 0800 555 111.