An audio tape and letters from a hoaxer who diverted attention away from the real Yorkshire Ripper have been misplaced, police have confirmed.
Peter Sutcliffe killed 13 women before being caught
The man fooled detectives into thinking he was the real killer at the height of the inquiry in 1978 and 1979.
Police switched their murder hunt from Yorkshire to Sunderland while Peter Sutcliffe murdered three more women.
On Friday, a West Yorkshire Police spokeswoman said the recordings which prompted the switch could not be found.
"There is currently an audit under way and at the moment we have been unable to find the original letters or tape," she said.
"The audit has been taking place for the last 12 months and it's as a result of that that we realised they were missing.
"There is no evidence that they have been stolen or disposed of maliciously or anything - we just cannot locate the items.
"We're going to have to check all our storage locations before we can say we definitely don't have them any more."
The hoaxer, who had an accent said to be from the Castletown area of Sunderland, sent police three letters and one tape, which was broadcast nationally in June 1979.
During that period Sutcliffe, from Bradford, was questioned and eliminated.
One of the reasons was that he did not have a Wearside accent.
The hoax, for which no-one was caught, led to years of additional police work costing more than £4m.
Sutcliffe was jailed for life in May 1981 after admitting the murders of 13 women and the attempted murder of seven others between 1978 and 1980.