Police forces in Scotland have confirmed that there are a total of 77 unsolved murders in their files.
The crimes include several notorious killings such as those carried out in Glasgow by the man known as Bible John.
But the oldest officially under investigation is the murder of Janet Henderson in Perth in 1866.
Forces said they never closed homicide investigations, with several "cold case" reviews carried out to take account of scientific developments.
BBC researchers asked all eight Scottish forces, under Freedom of Information legislation, for the number of unsolved murders on their files.
Cold case review
Most only went back to 1975 when the current force structure was established, but Tayside Police gave details of two from before the First World War.
They were the murders of Janet Henderson at a farm in Forgandenny near Perth in 1866, and Jean Milne, whose body was found in a house in Broughty Ferry in 1912.
In a statement, the force said: "Unresolved homicides or murders are never closed and they remain open, should any new evidence be made known to police.
"In addition, all unresolved homicides are the subject of periodic review in an effort to identify new evidence and are also reviewed in light of recent scientific and forensic advances.
"This ensures that every possible effort is made to trace the offender in each case."
Scotland's largest force, Strathclyde Police, said it had 53 unsolved homicides, although only crimes from 1975 are retained on a central database.
It offered details of some earlier killings as a result of a "cold case" review ordered by the Association of Chief Police Officers in Scotland (ACPOS) into unsolved murders of women.
These include the three victims presumed to have been killed by the man known as Bible John in the late 1960s.
The ACPOS review was headed by Tom Wood, then Deputy Chief Constable of Lothian and Borders Police.
One of the cases it investigated was the death of two women last seen at the World's End pub in the Royal Mile in Edinburgh in 1979.
It led to the trial of Angus Sinclair, but he was acquitted after the judge ruled there was no case to answer, so the cases remain unsolved.
A further case, the death of prostitute Sheila Anderson in April 1983, is currently under "active" review.
Central Scotland Police and Dumfries and Galloway Constabulary each reported they had no unsolved murders.