The body of Robert Lane, 50, was found in a house on Drage Street on Thursday.
Post-mortem tests were inconclusive and a 35-year-old man arrested over the death has been released on police bail.
Mr Lane stood unsuccessfully in the 2015 general election for North East Derbyshire and intended to run for police commissioner but failed to file papers in time.
Police said further tests were being carried out in an effort to determine the cause of death.
Mr Lane taught at secondary schools across the country for more than 20 years and was a freelance writer.
A man who died in unexplained circumstances in Derby has been named as a former parliamentary candidate.