A Vancouver Police Department internal review into convicted killer Robert Pickton's case file said the women went missing after the city's police force first forwarded information about Pickton to the Royal Canadian Mounted Police in 1998.

The report comes three weeks after Canada's top court denied the final appeal of the farmer convicted in 2007 of butchering six women and feeding them to pigs at his farm in the Vancouver, British Columbia, suburb of Port Coquitlam. He was also charged in 20 other killings, but those charges never went to trial.

Police have said it is Canada's worst serial murder case.

Pickton had confessed to an undercover police officer planted in his cell that he had killed 49 women and intended to keep killing.

He was arrested for attempted murder in 1997. The charges involving a drug-addicted prostitute, who was handcuffed by Pickton during the attack, were dropped because prosecutors said the woman was not a credible witness.

However, both Pickton and the woman were severely injured in the attack and wound up in the same hospital. Medical staff operating on the woman found the key to the handcuffs she still had on, in Pickton's pocket.

Pickton was convicted in the death of Marnie Frey, who was last heard from in 1997. Her aunt, Joyce Lachance, said the family told police about Pickton's farm in the spring of 1998.

In mid-July 1999, police were told that a woman called Lynn Ellingsen had helped Pickton pick up a prostitute and take her to his property.

She said she had seen "Pickton hang the women by the neck in his barn, cut strips of flesh off her legs and murder her," the report says.

A man whose name has been withheld in the report, told police in 1999, "he had observed handcuffs in Pickton's bed and a 'special' freezer from which (he) was served meat that he came to believe was human," the report said.

The 450-page report blames the Vancouver police force and the federal police body for errors that prevented either police force from arresting Pickton after he had been tagged as a prime suspect in 1999 as police investigated reports of missing sex workers.

The report said Pickton was interviewed by the RCMP in 2000, and the farmer consented to a search of his land, but no search took place.

The RCMP eventually got onto the property in 2002 as a result of a firearms warrant. During that search, they found identifications belonging to some of the missing women. They then conducted a following search under a murder warrant and uncovered the remains and DNA of dozens of women.

"The case broke because of serendipitous circumstances," said the report.

There have long been questions about why police didn't take reports of women disappearing from Vancouver more seriously. Some have suggested it is because most of the women Pickton is accused of murdering are prostitutes or drug addicts.

Pickton and his brother used to throw parties at the hog farm in a barn they dubbed "Piggy's Palace." Investigators have said they were drunken parties with prostitutes and plenty of drugs.

The report was leaked to the media two weeks in advance of its due date, so the RCMP said Friday it needs further time to review it.

"The RCMP deeply regrets that we weren't able to gather the evidence necessary to lay a charge sooner, and we welcome independent insight into our actions at that time," said Deputy Commissioner Gary Bass.

The report makes a series of recommendations including improving information-sharing between police forces.

After Pickton's arrest, authorities searched his suburban Vancouver farm. At his trial, investigators testified that they found body parts, blood samples, bone fragments and the belongings of many women.

The severed heads of Mona Wilson, Sereena Abotsway and Andrea Joesbury were found on Pickton's farm. Each woman had been shot.