The parents of murdered Hanwell schoolgirl Alice Gross said they are "extremely concerned, bewildered and angry" after a coroner left a file on the investigation into their daughter's death on a train.
Rose Hodgkiss and Jose Gross are furious after it was revealed that the senior west London coroner, Chinyere Inyama, lost the 30-page police file in November 2014, a month after Alice's body was found.
Police tried to recover the file, which contained evidence against prime suspect Arnis Zalkalns, but concluded it had probably been "destroyed as waste".
The 14-year-old's parents have demanded to know why they were not told.
They said, in a statement: "We have looked to the police and coroner to help us through our awful loss. Yet now we learn they - either independently or together - have withheld from us the loss of this terribly sensitive information about Alice.
"We are extremely concerned, bewildered and angry - and we have asked for a full written explanation as to what exactly happened and why we were not told."
The Ministry of Justice is now investigating the file's loss and why it was ever taken from the coroner's office.
Alice disappeared last August, sparking Scotland Yard's biggest search operation since the July 7 London bombings.
But the teenager's body was found in the Grand Union Canal, in Ealing, on September 30.
Days later Zalkalns, 41, was found hanged in Boston Manor Park, and police later said the Latvian - who had previously served seven years for murdering his wife in his home country - would have been charged with Alice's murder if he had still been alive.
An inquest into Alice's death was opened and adjourned in October, but the coroner lost the vital document - given to him to help with preparations for the inquests into the deaths of Alice and Zalkalns - a month later.
A spokesman for the Metropolitan Police said: "In November 2014, the MPS was informed by HM Coroner London West that he had inadvertently disposed of a single document relating to the police evidence against Arnis Zalkalns.
"An investigation to recover it was undertaken. This concluded that it was highly likely it had been destroyed as waste."
Details of the evidence against Zalkalns were given to the media in January with the agreement of the Crown Prosecution Service and the coroner.
The Ministry of Justice said of the loss: "This clearly appears to be a troubling incident. A full investigation is now under way."
A full inquest into Alice's death is due to be held later this year and, at a pre-inquest review earlier this month, Mr Inyama reserved judgment on whether it will be a jury inquest.
He is expected to hand down his ruling on the scope of the inquest in the next week.